WNR Western Refining Inc. Western Refining, Inc.

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WESTERN REFINING, INC.

NOTICE OF THE SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON MARCH 24, 2017

AT 1500 NORTH PRIEST DRIVE

TEMPE, AZ 85281

NOTICE IS HEREBY GIVEN that a special meeting of stockholders of Western Refining, Inc., which is referred to as Western Refining, will be held on March 24, 2017, at 10:00 a.m., Mountain Standard Time, at 1500 North Priest Drive, Tempe, AZ 85281, for the following purposes:

Western Refining stockholder approval of the merger proposal is required to complete the merger. Western Refining stockholders will also be asked to approve the Western Refining adjournment proposal, the non-binding compensation advisory proposal and the non-binding certificate of incorporation amendment proposal. Western Refining will transact no other business at the Western Refining special meeting. The record date for the Western Refining special meeting has been set as February 10, 2017. Only Western Refining stockholders of record as of the close of business on such record date are entitled to notice of, and to vote at, the Western Refining special meeting or any adjournments and postponements thereof. See the section entitled “ Special Meeting of Western Refining Stockholders ” beginning on page 64 of the joint proxy statement/prospectus accompanying this notice for additional information.

The Western Refining board unanimously recommends that holders of Western Refining common stock vote “FOR” the merger proposal, “FOR” the Western Refining meeting adjournment proposal if necessary or appropriate to solicit additional proxies in favor of such adoption, “FOR” the non-binding compensation advisory proposal and “FOR” the non-binding certificate of incorporation amendment proposal.

The Western Refining stockholder proposals are described in more detail in the accompanying joint proxy statement/prospectus, which you should read carefully in its entirety before you vote. A copy of the merger agreement is attached as Annex A to the accompanying joint proxy statement/prospectus and copies of the voting agreements are attached as Annexes B, C and D to the accompanying joint proxy statement/prospectus.

PLEASE VOTE AS PROMPTLY AS POSSIBLE, WHETHER OR NOT YOU PLAN TO ATTEND THE WESTERN REFINING SPECIAL MEETING. IF YOU LATER DESIRE TO REVOKE OR CHANGE YOUR PROXY FOR ANY REASON, YOU MAY DO SO IN THE MANNER DESCRIBED IN

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THE ACCOMPANYING JOINT PROXY STATEMENT/PROSPECTUS. FOR FURTHER INFORMATION CONCERNING THE PROPOSALS BEING VOTED UPON, USE OF THE PROXY AND OTHER RELATED MATTERS, YOU ARE URGED TO READ THE ACCOMPANYING JOINT PROXY STATEMENT/PROSPECTUS.

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REFERENCES TO ADDITIONAL INFORMATION

This joint proxy statement/prospectus incorporates important business and financial information about Tesoro Corporation, which is referred to as Tesoro, and Western Refining, Inc., which is referred to as Western Refining, from other documents that Tesoro and Western Refining have filed with the U.S. Securities and Exchange Commission, which is referred to as the SEC, and that are contained in or incorporated by reference herein. For a listing of documents incorporated by reference herein, please see the section entitled “ Where You Can Find More Information ” beginning on page 211. This information is available for you to review free of charge at the SEC’s public reference room located at 100 F Street, N.E., Room 1580, Washington, DC 20549, and through the SEC’s website at www.sec.gov .

You may request copies of this joint proxy statement/prospectus and any of the documents incorporated by reference herein or other information concerning Tesoro or Western Refining, without charge, upon written or oral request to the applicable company’s principal executive offices. The respective addresses and telephone numbers of such principal executive offices are listed below.

To obtain timely delivery of these documents before Tesoro’s special meeting of stockholders, Tesoro stockholders must request the information no later than March 10, 2017.

To obtain timely delivery of these documents before Western Refining’s special meeting of stockholders, Western Refining stockholders must request the information no later than March 17, 2017.

In addition, if you have questions about the merger or the accompanying joint proxy statement/prospectus, would like additional copies of the joint proxy statement/prospectus or need to obtain proxy cards or other information related to the proxy solicitation, please contact Innisfree M&A Incorporated, the proxy solicitor for Tesoro, toll-free at 1-888-750-5834 or collect at 1-212-750-5833, or MacKenzie Partners, Inc., the proxy solicitor for Western Refining, toll-free at 1-800-322-2885 or collect at 1-212-929-5500. You will not be charged for any of these documents that you request.

ABOUT THIS JOINT PROXY STATEMENT/PROSPECTUS

This document, which forms part of a registration statement on Form S-4 filed with the SEC by Tesoro (File No. 333-215080), constitutes a prospectus of Tesoro under Section 5 of the Securities Act of 1933, as amended, which is referred to as the Securities Act, with respect to the shares of common stock of Tesoro, which is referred to as Tesoro common stock, to be issued to Western Refining stockholders pursuant to the Agreement and Plan of Merger, dated as of November 16, 2016, as such agreement may be amended from time to time, which is referred to as the merger agreement, among Western Refining, Tesoro, Tahoe Merger Sub 1, Inc., which is referred to as Merger Sub 1, and Tahoe Merger Sub 2, LLC, which is referred to as Merger Sub 2, as it may be amended from time to time. This document also constitutes a proxy statement of each of Tesoro and Western Refining under Section 14(a) of the Securities Exchange Act of 1934, as amended, which is referred to as the Exchange Act.

Tesoro has supplied all information contained or incorporated by reference herein relating to Tesoro, and Western Refining has supplied all information contained or incorporated by reference herein relating to Western Refining. Tesoro and Western Refining have both contributed to the information relating to the merger contained in this joint proxy statement/prospectus.

You should rely only on the information contained in or incorporated by reference herein in connection with any vote, the giving or withholding of any proxy or any investment decision in connection with the merger. Tesoro and Western Refining have not authorized anyone to provide you with information that is different from that contained in or incorporated by reference herein. This joint proxy statement/prospectus is dated February 17, 2017, and you should not assume that the information contained in this joint proxy statement/prospectus is accurate as of any date other than such date unless otherwise specifically provided herein. Further, you should not assume that the information incorporated by reference herein is accurate as of any date other than the date of the incorporated document. Neither the mailing of this joint proxy statement/prospectus to Tesoro or Western Refining stockholders nor the issuance by Tesoro of shares of its common stock pursuant to the merger agreement will create any implication to the contrary.

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETINGS

The following are answers to certain questions that you may have regarding the Tesoro and Western Refining special meetings. Tesoro and Western Refining urge you to read carefully the remainder of this document because the information in this section may not provide all the information that might be important to you in determining how to vote. Additional important information is also contained in the appendices to, and the documents incorporated by reference in, this document.

Tesoro. The issuance of Tesoro common stock in connection with the merger must be approved by the stockholders of Tesoro in accordance with the rules of the New York Stock Exchange, which are referred to as the NYSE rules, in order for the merger to be consummated. Tesoro is holding a special meeting of its stockholders, which is referred to as the Tesoro special meeting, to obtain that approval. Tesoro stockholders will also be asked to vote on certain other matters described in this joint proxy statement/prospectus at the Tesoro special meeting and to approve the adjournment of the Tesoro special meeting, if necessary or appropriate, to solicit additional proxies in the event there are not sufficient votes at the time of the special meeting to approve the issuance of Tesoro common stock in connection with the merger.

Western Refining. The merger agreement must be adopted by the stockholders of Western Refining in accordance with the General Corporation Law of the State of Delaware, which is referred to as the DGCL, in order for the merger to be consummated. Western Refining is holding a special meeting of its stockholders, which is referred to as the Western Refining special meeting, to obtain that approval. Western Refining stockholders will also be asked to vote on certain other matters described in this joint proxy statement/prospectus at the Western Refining special meeting and to approve the adjournment of the Western Refining special meeting, if necessary or appropriate, to solicit additional proxies in the event there are not sufficient votes at the time of the special meeting to adopt the merger agreement.

Western Refining. The Western Refining special meeting will be held at 10:00 a.m., Mountain Standard Time, on March 24, 2017, at 1500 North Priest Drive, Tempe, AZ 85281.

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Western Refining. The stockholders of Western Refining will be asked to:

Western Refining. Yes. The merger cannot be completed unless the merger agreement is adopted by the holders representing a majority of the outstanding shares of Western Refining common stock entitled to vote thereon at the Western Refining special meeting. Only Western Refining stockholders as of the close of business on the record date are entitled to vote at the Western Refining special meeting. The board of directors of Western Refining, which is referred to as the Western Refining board or the Western Refining board of directors, unanimously recommends that such Western Refining stockholders vote “ FOR ” for the approval of the merger proposal, “ FOR ” the approval of the Western Refining adjournment proposal, “ FOR” the approval of the non-binding compensation advisory proposal and “ FOR ” the approval of the non-binding certificate of incorporation amendment proposal.

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The Tesoro certificate of incorporation amendment proposal. Approval of the Tesoro certificate of incorporation amendment proposal requires the affirmative vote of a majority of the shares of Tesoro common stock outstanding as of the close of business on the record date and entitled to vote on the proposal. Abstentions and broker non-votes will have the same effect as a vote “AGAINST” the proposal.

Tesoro adjournment proposal . Approval of the Tesoro adjournment proposal requires the affirmative vote of a majority of the shares of Tesoro common stock present in person or by proxy at the Tesoro special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

The Western Refining adjournment proposal. Approval of the Western Refining adjournment proposal requires the affirmative vote of a majority of the shares of Western Refining common stock present in person or by proxy at the Western Refining special meeting and entitled to vote on the proposal, regardless of whether a quorum is present. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

The Western Refining non-binding compensation advisory proposal on specified compensation . Approval of the non-binding compensation advisory proposal requires the affirmative vote of a majority of the shares of Western Refining common stock present in person or by proxy at the Western Refining special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

The Western Refining non-binding certificate of incorporation amendment proposal approving the amendment of Tesoro’s certificate of incorporation . Approval of the non-binding certificate of incorporation amendment proposal requires the affirmative vote of a majority of the shares of Western Refining common stock present in person or by proxy at the Western Refining special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

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Elections to receive stock consideration, which are referred to as stock elections, are not subject to proration. Elections to receive cash consideration, which are referred to as cash elections, are subject to the adjustment and proration procedures set forth in the merger agreement to ensure that the total number of shares of Western Refining common stock converted into cash consideration will not exceed 10,843,042 shares of Western Refining common stock less the number of excluded shares (as defined in the section entitled “ The Merger—Consideration to Western Refining Stockholders ” beginning on page 71), which is referred to as the cash election number.

If you elect to receive stock consideration and would otherwise be entitled to receive a fractional share of Tesoro common stock (taking into account all Western Refining shares for which you have elected or been deemed to have elected to receive stock consideration), you will receive cash in lieu of such fractional share, and you will not be entitled to dividends, voting rights or any other rights in respect of such fractional share. For more information regarding adjustment and proration procedures, see the section entitled “ The Merger Agreement—Merger Consideration ” beginning on page 140.

For more information regarding the stock consideration or cash consideration, as applicable, to be provided to Western Refining stockholders, referred to as the per share merger consideration, see the section entitled “ The Merger—Consideration to Western Refining Stockholders ” beginning on page 71. For more information regarding election mechanics, see the section entitled “ The Merger Agreement—Election and Exchange Procedures ” beginning on page 143.

The number of cash election shares of a holder of Western Refining common stock that are to remain cash election shares pursuant to the calculation described above will be rounded downward where needed. The calculations described above will be performed by American Stock Transfer & Trust Company, Tesoro’s and Western Refining’s transfer agent, referred to as the exchange agent.

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Vested Western Refining Restricted Stock Units

Each outstanding Western Refining restricted stock unit, which is referred to as a Western Refining RSU, that is vested as of the effective time of the first merger (after application of any vesting acceleration provisions) will automatically be cancelled and converted into, as soon as reasonably practicable after the effective time of the first merger, an amount in cash (without interest) equal to the number of shares of such vested Western Refining RSU immediately prior to the effective time of the first merger multiplied by the cash consideration, less applicable taxes required to be withheld with respect to such payment.

Unvested Western Refining Restricted Stock Units

Each outstanding Western Refining RSU that is not vested as of the effective time of the first merger (after application of any vesting acceleration provisions) will automatically be converted into an RSU denominated in shares of Tesoro common stock, which is referred to as a Tesoro RSU, covering a number of shares of Tesoro common stock, rounded down to the nearest whole number, equal to the number of shares of Western Refining common stock subject to such Western Refining RSU immediately prior to the effective time of the first merger multiplied by the exchange ratio. Except as specifically provided in the merger agreement, following the effective time of the first merger, each such Tesoro RSU will continue to be governed by the same terms and conditions (including vesting terms) as were applicable to the applicable Western Refining RSU immediately prior to the effective time of the first merger.

Western Refining Performance Unit Awards

Each outstanding Western Refining performance unit award, which is referred to as a Western Refining PUA, whether vested or unvested, will remain a performance unit award denominated in the same cash value as the Western Refining PUA, except that any references in such Western Refining PUA to shares of Western Refining common stock will be to shares of Tesoro common stock, which is referred to as a Tesoro PUA. Except as provided in the merger agreement, following the effective time of the first merger, each such Tesoro PUA will continue to be governed by the same terms and conditions (including vesting terms) as were applicable to the applicable Western Refining PUA immediately prior to the effective time of the first merger, provided that the performance metrics will be adjusted by Western Refining with the approval of Tesoro to reflect the first merger.

Other Company Awards

Any other right to acquire or receive shares of Western Refining common stock and each award consisting of shares of Western Refining common stock will automatically be converted into the right to acquire or receive benefits measured by the value of (as the case may be) the number of shares of Tesoro common stock, rounded down to the nearest whole number, equal to the number of shares of Western Refining common stock subject to such award immediately prior to the effective time of the first merger multiplied by the exchange ratio, and to the extent such award provides for payments to the extent the value of the shares of Western Refining common stock exceeds a specified reference price, at a reference price per share, rounded to the nearest whole cent, equal to the reference price per share of Western Refining common stock immediately prior to the effective time of the first merger divided by the exchange ratio. Except as specifically provided in the merger agreement, following the effective time of the first merger, each such right will otherwise be subject to the same terms and conditions (including, as applicable, vesting and exercisability terms) as were applicable to the rights under the relevant plan immediately prior to the effective time of the first merger, provided that any applicable performance metrics will be adjusted by Western Refining with the approval of Tesoro to reflect the first merger.

For more information regarding the treatment of Western Refining equity awards, see the section entitled “ The Merger Agreement—Treatment of Western Refining Stock Plans in the Merger ” beginning on page 142.

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Western Refining. The Western Refining board recommends that you vote “ FOR ” for the approval of the merger proposal, “ FOR ” the approval of the Western Refining adjournment proposal, “ FOR ” the approval of the non-binding compensation advisory proposal and “ FOR ” the approval of the non-binding certificate of incorporation amendment proposal. For more information regarding how the board of directors of Western Refining recommends that you vote, see the section entitled “ The Merger—Recommendation of Western Refining’s Board of Directors and Reasons for the Merger ” beginning on page 91.

Western Refining special meeting. The board of directors of Western Refining has fixed February 10, 2017 as the record date for the Western Refining special meeting. All holders of record of shares of Western Refining common stock as of the close of business on the record date are entitled to receive notice of, and to vote at, the Western Refining special meeting, provided that those shares remain outstanding on the date of the Western Refining special meeting. Physical attendance at the Western Refining special meeting is not required to vote. See the section entitled “ Questions and Answers About the Merger and the Special Meetings—How can I vote my shares without attending the special meeting? ” beginning on page 10 for instructions on how to vote your shares without attending the Western Refining special meeting.

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Tesoro stockholders. If you are a stockholder of record of Tesoro common stock as of the close of business on the record date, and you vote by phone, by Internet or by signing, dating and returning your proxy card in the enclosed postage-paid envelope, you designate two of Tesoro’s officers as your proxies at the Tesoro special meeting, each with full power to act without the other and with full power of substitution. These two officers are Carrie P. Ryan and Elisa D. Watts.

Western Refining stockholders. If you are a stockholder of record of Western Refining common stock as of the close of business on the record date, and you vote by phone, by Internet or by signing, dating and returning your proxy card in the enclosed postage-paid envelope, you designate two of Western Refining’s officers as your proxies at the Western Refining special meeting, each with full power to act without the other and with full power of substitution. These two officers are Scott D. Weaver and Lowry Barfield.

Western Refining stockholders. Each Western Refining stockholder of record is entitled to one vote for each share of Western Refining common stock held of record by him or her as of the close of business on the record date.

Quorum for Tesoro special meeting. A quorum will exist at the Tesoro special meeting with respect to each matter to be considered at the Tesoro special meeting if the holders of a majority of shares of Tesoro common stock outstanding as of the close of business on the record date are present in person or represented by proxy at the Tesoro special meeting.

Quorum for Western Refining special meeting. A quorum will exist at the Western Refining special meeting with respect to the matters to be considered at the Western Refining special meeting if the holders of a majority of shares of Western Refining common stock issued and outstanding and entitled to vote as of the close of business on the record date are present in person or represented by proxy at the Western Refining special meeting.

If either Tesoro or Western Refining provides notice that it elects in accordance with the merger agreement to effect the second merger, then immediately following the completion of the first merger, subject to

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certain conditions, the surviving corporation will merge with and into Merger Sub 2, which is referred to as the second merger. If the second merger is effected, the first merger and the second merger are referred to together in this joint proxy statement/prospectus as the merger. If the second merger occurs, the separate corporate existence of Western Refining will cease as a result and Merger Sub 2 will continue as the surviving company and a wholly owned subsidiary of Tesoro. If the second merger is not effected, the first merger is also referred to in this joint proxy statement/prospectus on its own as the merger.

The merger agreement does not require that an election to effect the second merger be made at any particular time prior to the effective time of the first merger.

At any time prior to the completion of the first merger, including after any or all of the Tesoro special meeting, the Western Refining special meeting and the election deadline, either Tesoro or Western Refining may, on the terms and subject to the conditions set forth in the merger agreement, provide notice to the other that it elects to effect the second merger. While Tesoro and Western Refining stockholders may not know whether the second merger will be effected when they vote on the proposals to be considered at the Tesoro special meeting and the Western Refining special meeting, respectively, or when Western Refining stockholders make their elections to receive the cash consideration or the stock consideration, an election to effect the second merger will not affect the amount of the per share merger consideration or the timing or manner in which Western Refining stockholders receive the per share merger consideration. Western Refining stockholders become entitled to receive the per share merger consideration at the effective time of the first merger, on the terms and subject to the conditions set forth in the merger agreement, irrespective of whether the second merger occurs.

If either Tesoro or Western Refining elects to effect the second merger, it will make a public announcement to that effect as promptly as practicable after providing notice thereof to the other party.

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Please note that if your shares are held in the Tesoro Corporation Thrift Plan, you may not vote in person at the Tesoro special meeting.

Failure to bring the appropriate documentation may delay your entry into or prevent you from attending the Tesoro special meeting. The doors to the meeting room will be closed promptly at the start of the meeting, and stockholders will not be permitted to enter after that time.

Western Refining. Shares of Western Refining common stock held directly in your name as the stockholder of record as of the close of business on February 10, 2017, the record date, may be voted in person at the Western Refining special meeting. If you choose to attend the Western Refining special meeting, you will need to bring valid, government-issued photo identification. If you are a beneficial owner of Western Refining common stock but not the stockholder of record of such shares of Western Refining common stock, you will also need proof of stock ownership to be admitted to the Western Refining special meeting. A recent brokerage statement or a letter from a bank or broker are examples of proof of ownership. Please note that if your shares are held in “street name” by a broker, bank or other nominee and you wish to vote at the Western Refining special meeting, you will not be permitted to vote in person unless you first obtain a legal proxy issued in your name from the record owner and present it to the inspector of election with your ballot at the Western Refining special meeting. To request a legal proxy, please contact your broker, bank or other nominee holder of record. It is suggested you do so in a timely manner to ensure receipt of your legal proxy prior to the Western Refining special meeting.

Failure to bring the appropriate documentation may delay your entry into or prevent you from attending the Western Refining special meeting. The doors to the meeting room will be closed promptly at the start of the meeting, and stockholders will not be permitted to enter after that time.

Western Refining. If you are a stockholder of record of Western Refining common stock as of the close of business on February 10, 2017, the record date, you can vote by proxy via the Internet, by telephone or by mail by following the instructions provided in the enclosed proxy card. Please note that if you are a beneficial owner, you may vote by submitting voting instructions to your bank, brokerage firm or other nominee, or otherwise by following instructions provided by your bank, brokerage firm or other nominee. Telephone and internet voting may be available to beneficial owner. Please refer to the vote instruction form provided by your bank, brokerage firm or other nominee.

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Western Refining. If your shares of Western Refining common stock are registered directly in your name with Western Refining’s transfer agent, American Stock Transfer & Trust Company, you are considered the stockholder of record with respect to those shares, and access to proxy materials is being provided directly to you. If your shares are held in a stock brokerage account or by a bank or other nominee, then you are considered the beneficial owner of those shares, which are considered to be held in street name. Access to proxy materials is being provided to you by your broker, bank or other nominee who is considered the stockholder of record with respect to those shares.

Direct holders (stockholders of record)

For shares of Tesoro and/or Western Refining common stock held directly, please complete, sign, date and return each proxy card (or cast your vote by telephone or Internet as provided on each proxy card) or otherwise follow the voting instructions provided in this joint proxy statement/prospectus in order to ensure that all of your shares of Tesoro and/or Western Refining common stock are voted.

Shares in “street name”

For shares of Tesoro and/or Western Refining common stock held in “street name” through a bank, brokerage firm or other nominee, you should follow the procedures provided by your bank, brokerage firm or other nominee to vote your shares.

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Western Refining. If you sign, date and return your proxy and do not indicate how you want your shares of Western Refining common stock to be voted, then your shares of Western Refining common stock will be voted “ FOR ” the approval of the merger proposal, “ FOR ” the approval of the Western Refining adjournment proposal, “ FOR ” the approval of the non-binding compensation advisory proposal and “ FOR ” the approval of the non-binding certificate of incorporation amendment proposal.

If you are a beneficial owner of Tesoro common stock as of the close of business on the record date, you must follow the instructions of your broker, bank or other nominee to revoke or change your voting instructions.

Western Refining . Yes. If you are a stockholder of record of Western Refining common stock as of the close of business on the record date, whether you vote by telephone, Internet or mail, you can change or revoke your proxy before it is voted at the meeting in one of the following ways:

If you are a beneficial owner of Western Refining common stock as of the close of business on the record date, you must follow the instructions of your broker, bank or other nominee to revoke or change your voting instructions.

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Assuming the merger qualifies as a reorganization, a stockholder of Western Refining generally will not recognize any gain or loss upon receipt of Tesoro common stock in exchange for Western Refining common stock in the merger, except that gain (but not loss) will be recognized in an amount not to exceed any cash received as part of the cash consideration (other than cash received in lieu of a fractional share) and gain or loss will be recognized with respect to any cash received in lieu of a fractional share of Tesoro common stock. The U.S. federal income tax consequences of the merger are discussed in more detail in the section entitled “ Material U.S. Federal Income Tax Consequences of the Merger ” beginning on page 172. The discussion of the material U.S. federal income tax consequences contained in this joint proxy statement/prospectus is intended to provide only a general discussion and is not a complete analysis or description of all potential U.S. federal income tax consequences of the merger that may vary with, or are dependent on, individual circumstances. In addition, it does not address the effects of any foreign, state or local tax laws.

TAX MATTERS ARE COMPLICATED AND THE TAX CONSEQUENCES OF THE MERGER WILL DEPEND ON THE FACTS OF YOUR OWN SITUATION. YOU SHOULD CONSULT YOUR OWN TAX ADVISOR AS TO THE SPECIFIC TAX CONSEQUENCES OF THE MERGER TO YOU IN YOUR PARTICULAR CIRCUMSTANCES.

Western Refining. Western Refining has retained MacKenzie Partners, Inc., which is referred to as MacKenzie, to assist in the solicitation process. Western Refining will pay MacKenzie a fee of approximately $50,000, as well as reasonable and documented out-of-pocket expenses. Western Refining also has agreed to indemnify MacKenzie against various liabilities and expenses that relate to or arise out of its solicitation of proxies (subject to certain exceptions).

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After receiving the proper documentation from you, following the effective time, the exchange agent will deliver to you the cash consideration or stock consideration (plus, in the case of stock consideration, any cash in lieu of fractional shares and any applicable dividends on Tesoro common stock with a record date after the merger is completed) to which you are entitled. More information on the documentation you are required to deliver to the exchange agent can be found in the section entitled “ The Merger Agreement—Election and Exchange Procedures ” beginning on page 143.

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For the past three years, Tesoro has paid a quarterly dividend on the Tesoro common stock as described in greater detail in the section entitled “ Comparative Per Share Market Price and Dividend Information—Tesoro Market Price and Dividend Information” beginning on page 39. Tesoro last declared a dividend on February 3, 2017, in an amount of $0.55 per Tesoro share, which will be paid on March 15, 2017 to Tesoro stockholders of record as of the close of business on February 28, 2017. Any future Tesoro dividends will remain subject to approval by the Tesoro board.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This registration statement on Form S-4, of which this joint proxy statement/prospectus forms a part, and the documents to which Western Refining and Tesoro refer you in this registration statement, of which this joint proxy statement/prospectus forms a part, as well as oral statements made or to be made by Western Refining and Tesoro, include certain “forward-looking statements” within the meaning of, and subject to the safe harbor created by, Section 27A of the Securities Act of 1933 and Section 21E of the Exchange Act, which are referred to as the safe harbor provisions. Words such as “may,” “will,” “could,” “would,” “anticipate,” “estimate,” “expect,” “predict,” “project,” “guidance,” “continue,” “future,” “potential,” “intend,” “plan,” “assume,” “believe,” “forecast,” “look,” “build,” “focus,” “create,” “work,” “continue” or the negative of such terms or other variations thereof and words and terms of similar substance used in connection with any discussion of future plans, actions, or events identify forward-looking statements with respect to the businesses, strategies and plans of Western Refining and Tesoro, their expectations relating to the merger and their future financial condition and performance. Tesoro and Western Refining caution investors that any forward-looking statements are subject to risks and uncertainties that may cause actual results and future trends to differ materially from those matters expressed in or implied by such forward-looking statements. Investors are cautioned not to place undue reliance on forward-looking statements. Among the risks and uncertainties that could cause actual results to differ from those described in forward-looking statements are the following:

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Such factors are difficult to predict and in many cases may be beyond the control of Tesoro and Western Refining. Tesoro’s and Western Refining’s forward-looking statements are based on assumptions that Tesoro and Western Refining, respectively, believe to be reasonable but that may not prove to be accurate. Consequently, all of the forward-looking statements Tesoro and Western Refining make in this document are qualified by the information contained or incorporated by reference herein, including the information contained under this heading and the information detailed in Tesoro’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015 and Tesoro’s Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2016, June 30, 2016 and September 30, 2016, and in Western Refining’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015 and Western Refining’s Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2016, June 30, 2016 and September 30, 2016. See the section entitled “ Where You Can Find More Information ” beginning on page 211.

Tesoro and Western Refining undertake no obligation to publicly release the result of any revisions to any such forward-looking statements that may be made to reflect events or circumstances that occur, or which they become aware of, except as required by applicable law or regulation. Readers are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference herein, including the matters addressed in the section entitled “Cautionary Statement Regarding Forward-Looking Statements” beginning on page 42, Western Refining stockholders should carefully consider the following risks before deciding how to vote with respect to the proposals to be considered and voted on at the Western Refining special meeting, and Tesoro stockholders should carefully consider the following risks before deciding how to vote with respect to the proposals to be considered and voted on at the Tesoro special meeting. Western Refining and Tesoro stockholders should also consider the other information in this joint proxy statement/prospectus and the other documents incorporated by reference herein, particularly the risk factors contained in Tesoro’s and Western Refining’s Annual Reports on Form 10-K and Quarterly Reports on Form 10-Q. See the section entitled “Where You Can Find More Information” beginning on page 211.

Risks Relating to the Merger

Because the market price of shares Tesoro common stock may fluctuate, Western Refining stockholders cannot be certain of the precise value of any stock consideration they may receive in the merger.

At the time the merger is completed, each issued and outstanding share of Western Refining common stock (except for excluded shares as defined in the section entitled “ The Merger—Consideration to Western Refining Stockholders ” beginning on page 71) will be converted into the right to receive either the cash consideration of $37.30, if the Western Refining stockholder makes a cash election (subject to adjustment and proration), or the stock consideration of 0.4350 of a share of Tesoro common stock, if the Western Refining stockholder makes or is deemed to have made a stock election. The cash election is subject to the adjustment and proration procedures set forth in the merger agreement. The exchange ratio for the stock consideration is fixed, and there will be no adjustment to the stock consideration for changes in the market price of shares of Tesoro common stock or Western Refining common stock prior to the completion of the merger. If the merger is completed, there will be a time lapse between each of the date of this joint proxy statement/prospectus, the dates on which Western Refining stockholders vote to approve the Western Refining merger proposal at the Western Refining special meeting and Tesoro stockholders vote to approve the Tesoro issuance proposal at the Tesoro special meeting, and the date on which Western Refining stockholders entitled to receive the stock consideration actually receive such stock consideration. The market value of shares of Tesoro common stock may fluctuate during and after these periods as a result of a variety of factors, including general market and economic conditions, changes in Tesoro’s businesses, operations and prospects and regulatory considerations. Such factors are difficult to predict and in many cases may be beyond the control of Tesoro and Western Refining. Consequently, at the time Western Refining stockholders must decide whether to adopt the merger agreement, and make their elections to receive the cash consideration or stock consideration, they will not know the actual market value of any stock consideration they will receive when the merger is completed. The actual value of any stock consideration received by Western Refining stockholders at the completion of the merger will depend on the market value of the shares of Tesoro common stock at that time. This market value may differ, possibly materially, from the market value of shares of Tesoro common stock at the time the merger agreement was entered into or at any other time. Western Refining stockholders should obtain current stock quotations for shares of Tesoro common stock before voting their shares of Western Refining common stock. For additional information about the Western Refining per share merger consideration, see the section entitled “ The Merger Agreement—Merger Consideration ” beginning on page 140.

The value of the cash consideration and the stock consideration may differ considerably. Additionally, in the event the cash consideration is worth more than the stock consideration, Western Refining stockholders may not be able to realize the full excess of the value of the cash consideration over the stock consideration due to prorating.

Since the cash consideration is a fixed amount of $37.30 and the market value of the stock consideration will continuously change with the market price for Tesoro common stock, it is possible that, at the time of the

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Tesoro special meeting, the election deadline, the effective time of the first merger and/or any other time, the cash consideration could be worth more, less or the same amount as the stock consideration. Further, because you will not be able to change your election after the election deadline (and because of the adjustment and proration procedures discussed below), you may receive per share merger consideration that is worth less than the other per share merger consideration you could have elected, even if the per share merger consideration you elected to receive is worth more at the election deadline than the per share merger consideration you actually receive. Additionally, since cash elections are subject to proration, in the event the cash consideration is worth more than the stock consideration, Western Refining stockholders may not be able to realize the excess of such difference even if they make a cash election with respect to all of their shares of Western Refining common stock.

The market price of shares of Tesoro common stock will continue to fluctuate after the merger.

Upon completion of the merger, holders of Western Refining common stock who receive stock consideration will become holders of shares of Tesoro common stock. The market price of shares of Tesoro common stock may fluctuate significantly following completion of the merger and holders of Western Refining common stock could lose some or all of the value of their investment in Tesoro common stock. In addition, the stock market has experienced significant price and volume fluctuations in recent times which, if they continue to occur, could have a material adverse effect on the market for, or liquidity of, the Tesoro common stock, regardless of Tesoro’s actual operating performance.

Western Refining stockholders may receive a form of consideration different from what they elect.

Although each Western Refining stockholder may elect, for each share of Western Refining common stock he or she owns that is issued and outstanding as of immediately prior to the effective time of the first merger, to receive the cash consideration or the stock consideration in the first merger, the aggregate amount of cash consideration to be paid in the first merger is subject to the adjustment and proration procedures set forth in the merger agreement to ensure that the total number of shares of Western Refining stock converted into the right to receive the cash consideration in the first merger will not exceed 10,843,042 shares of Western Refining stock issued and outstanding immediately prior to the effective time less the number of excluded shares (as defined in the section entitled “ The Merger—Consideration to Western Refining Stockholders ” beginning on page 71 and in the merger agreement), which is referred to as the cash election number. As a result, if Western Refining stockholders make a cash election in respect of more than the cash election number of shares of Western Refining common stock, which is referred to as an oversubscription of the cash election, Western Refining stockholders who have elected cash consideration may instead receive stock consideration in respect of some or all of the shares of Western Refining common stock in respect of which they have made cash elections. In some cases, due to rounding, the pro rata portion may be zero, such that none of such shares of Western Refining common stock will be converted into the right to receive the cash consideration. This could result in, among other things, tax consequences that differ from those that would have resulted if such stockholders had received the form of consideration that they had elected. For a more detailed description of how the proration and adjustment procedures would work in the event that there is an oversubscription of the cash election, see the section entitled “ The Merger Agreement—Merger Consideration ” beginning on page 140.

After a Western Refining stockholder has made an election in respect of his or her shares of Western Refining common stock, he or she will not be able to sell those shares unless he or she revokes his or her election prior to the election deadline or the merger agreement is terminated.

To be effective, a form of election must be properly completed, signed and submitted to the exchange agent by 5:00 PM (Eastern Time) on the business day that is two trading days prior to the closing date for the first merger, or such other date and time as Tesoro may publicly announce with the consent of Western Refining. The closing date of the first merger will be publicly announced by Tesoro at least four business days prior to the anticipated closing date of the first merger. After a Western Refining stockholder has submitted a form of election, under the terms of the election, he or she will not be able to sell any Western Refining shares covered

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by his or her form of election, regardless of whether those Western Refining shares are held in certificated or book-entry form, unless he or she revokes his or her election before the deadline by written notice received by the exchange agent prior to the election deadline. While the parties have agreed to establish an election deadline that is a relatively short time before the anticipated completion date of the first merger, there can be no assurance that unforeseen circumstances will not cause the completion of the first merger to be delayed after the deadline has been established.

Western Refining stockholders will have a reduced ownership and voting interest in the combined company after the merger and will exercise less influence over management.

Currently, Western Refining stockholders have the right to vote in the election of the Western Refining board and the power to approve or reject any matters requiring stockholder approval under Delaware law and Western Refining’s certificate of incorporation and bylaws. Upon completion of the merger, each Western Refining stockholder who receives shares of Tesoro common stock in the merger will become a stockholder of Tesoro with a percentage ownership of Tesoro that is smaller than the Western Refining stockholder’s current percentage ownership of Western Refining. Based on the number of issued and outstanding shares of Tesoro common stock and shares of Western Refining common stock as of February 8, 2017 and on the exchange ratio of 0.4350, after the merger Western Refining stockholders are expected to become owners of between approximately 26.93% and approximately 29.02% of the outstanding shares of Tesoro common stock, depending on the number of Western Refining stockholders that make a cash election, and without giving effect to any shares of Tesoro common stock held by Western Refining stockholders prior to the completion of the merger. Even if all former Western Refining stockholders voted together on all matters presented to Tesoro stockholders from time to time, the former Western Refining stockholders would exercise significantly less influence over Tesoro after the completion of the merger relative to their influence over Western Refining prior to the completion of the merger, and thus would have a less significant impact on the approval or rejection of future Tesoro proposals submitted to a stockholder vote.

Shares of Tesoro common stock received by Western Refining stockholders as a result of the merger will have different rights from shares of Western Refining common stock.

Upon completion of the merger, Western Refining stockholders will no longer be stockholders of Western Refining, and Western Refining stockholders who receive stock consideration will become stockholders of Tesoro. There will be important differences between the current rights of Western Refining stockholders and the rights to which such stockholders will be entitled as stockholders of Tesoro. See the section entitled “ Comparison of Stockholders’ Rights ” beginning on page 191 for a discussion of the different rights associated with the shares of Tesoro common stock.

The market price of shares of Tesoro common stock may be affected by factors different from those that historically have affected shares of Western Refining common stock.

Upon completion of the merger, holders of Western Refining common stock who receive stock consideration will become holders of Tesoro common stock. The businesses of Tesoro differ from those of Western Refining in certain respects, and, accordingly, the financial position or results of operations and/or cash flows of Tesoro after the merger, as well as the market price of shares of Tesoro common stock, may be affected by factors different from those currently affecting the financial position or results of operations and/or cash flows of Western Refining. Following the completion of the merger, Western Refining will be part of a larger company with other lines of business and a broader geographic footprint, so decisions affecting Western Refining may be made in respect of the larger combined business as a whole rather than the Western Refining businesses individually. For a discussion of the businesses of Tesoro and Western Refining and of some important factors to consider in connection with those businesses, see the section entitled “ Information About the Companies ” beginning on page 56, and the documents incorporated by reference in the section entitled “ Where You Can Find More Information ” beginning on page 211, including, in particular, in the sections entitled “Risk Factors” in each

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of Tesoro’s Annual Report on Form 10-K for the year ended December 31, 2015 and Western Refining’s Annual Report on Form 10-K for the year ended December 31, 2015.

The merger agreement limits Tesoro’s ability and Western Refining’s ability to pursue alternatives to the merger.

The merger agreement contains provisions that may discourage a third party from submitting an acquisition proposal to Tesoro or Western Refining that might result in greater value to Tesoro’s or Western Refining’s respective stockholders than the merger, or may result in a potential acquirer of Tesoro, or a potential competing acquirer of Western Refining, proposing to pay a lower per share price to acquire Tesoro or Western Refining, respectively, than it might otherwise have proposed to pay. These provisions include a general prohibition on Tesoro and Western Refining from soliciting or, subject to certain exceptions relating to the exercise of fiduciary duties by the Tesoro board or the Western Refining board, entering into discussions with any third party regarding any acquisition proposal or offer for a competing transaction. Western Refining also has an unqualified obligation to submit the Western Refining merger proposal to a vote by its stockholders, even if Western Refining receives an alternative acquisition proposal that the Western Refining board believes is superior to the merger, and Tesoro has an unqualified obligation to submit the Tesoro issuance proposal to a vote by its stockholders, even if Tesoro receives an alternative acquisition proposal that the Tesoro board believes is superior to the merger, in each case unless Western Refining or Tesoro, as applicable, terminates the merger agreement in accordance with its terms prior to such time. See the section entitled “ The Merger Agreement—Termination ” beginning on page 165.

The merger agreement may be terminated in accordance with its terms and the merger may not be completed.

The merger agreement is subject to a number of conditions that must be fulfilled in order to complete the merger. Those conditions include, among others: the adoption of the merger agreement by Western Refining stockholders, the approval by Tesoro stockholders of the issuance of shares of Tesoro common stock in connection with the merger, the expiration or termination of the waiting period applicable to the merger under the HSR Act without the imposition of a burdensome condition (as defined in the section entitled “The Merger Agreement—Reasonable Best Efforts ; Regulatory Filings and Other Actions ” beginning on page 159 and in the merger agreement), the accuracy of representations and warranties under the merger agreement (subject to the materiality standards set forth in the merger agreement), Tesoro’s and Western Refining’s performance of their respective obligations under the merger agreement in all material respects and Western Refining’s receipt of a written opinion of Davis Polk & Wardwell LLP (or another nationally recognized law firm selected by Western Refining) regarding certain U.S. federal income tax matters. These conditions to the closing of the merger may not be fulfilled in a timely manner or at all, and, accordingly, the merger may be delayed or may not be completed.

In addition, if the merger is not completed by November 16, 2017, either Tesoro or Western Refining may choose not to proceed with the merger, and the parties can mutually decide to terminate the merger agreement at any time, before or after stockholder approval. In addition, Tesoro and Western Refining may elect to terminate the merger agreement in certain other circumstances. See the section entitled “ The Merger Agreement—Termination ” beginning on page 165.

Failure to complete the merger could negatively impact the price of shares of Tesoro common stock and the price of shares of Western Refining common stock, as well as Tesoro’s and Western Refining’s respective future businesses and financial results.

The merger agreement contains a number of conditions that must be satisfied or waived prior to the completion of the merger. There can be no assurance that all of the conditions to the merger will be so satisfied or waived. If the conditions to the merger are not satisfied or waived, Tesoro and Western Refining will be unable to complete the merger.

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If the merger is not completed for any reason, including the failure to receive the required approvals of Tesoro’s and Western Refining’s respective stockholders, Tesoro’s and Western Refining’s respective businesses and financial results may be adversely affected as follows:

In addition to the above risks, if the merger agreement is terminated and either party’s board seeks an alternative transaction, such party’s stockholders cannot be certain that such party will be able to find a party willing to engage in a transaction on more attractive terms than the merger. If the merger agreement is terminated under specified circumstances, either Tesoro or Western Refining may be required to pay the other party a termination fee, reverse termination fee or other termination-related payment. See the section entitled “ The Merger Agreement—Termination ” beginning on page 165 for a description of these circumstances.

Required regulatory approvals may not be received, may take longer than expected to be received or may impose conditions that are not presently anticipated or cannot be met. In addition, an adverse outcome of any antitrust or similar review undertaken by a governmental authority could prevent the merger from being completed or have an adverse effect on Tesoro following the merger.

Completion of the merger is conditioned upon the approval by the NYSE of the listing of the shares of Tesoro common stock to be issued in the merger upon official notice of issuance and the expiration or termination of the waiting period applicable to the merger under the HSR Act. Although each party has agreed to use its respective reasonable best efforts to obtain the requisite stock exchange and governmental approvals, there can be no assurance that these approvals will be obtained and that the other conditions to completing the merger will be satisfied. In addition, the governmental authorities from which the regulatory approvals are required may impose conditions on the completion of the merger or require changes to the terms of the merger or merger agreement. Such conditions or changes and the process of obtaining regulatory approvals could have the effect of delaying or impeding completion of the merger or of imposing additional costs or limitations on Tesoro following completion of the merger, any of which might have an adverse effect on Tesoro following completion of the merger.

Each party’s obligation to complete the merger is subject to the condition that the expiration or termination of the waiting period applicable to the merger under the HSR Act be obtained without the imposition of a burdensome condition, as defined in the section entitled “ The Merger Agreement—Reasonable Best Efforts; Regulatory Filings and Other Actions ” beginning on page 159 and in the merger agreement.

As further described in the section entitled “ Litigation and Regulatory Reviews/Investigations Related to the Merger” beginning on page 130, Tesoro and Western Refining have each received a substantially identical letter, dated January 27, 2017, from the Minnesota AG stating that the Office of the Minnesota AG intends to review the merger under the antitrust laws and requesting that Tesoro and Western Refining provide information regarding the potential competitive effects of the merger in Minnesota. While the completion of such review, or

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of any other antitrust or similar review undertaken by a governmental authority (other than for the expiration or termination of the waiting period applicable to the merger pursuant to the HSR Act) is not a direct condition to the completion of the merger, one of the conditions to the closing of the merger is that there must not have been enacted, issued, promulgated, enforced or entered by a court or other governmental entity of competent jurisdiction any applicable law that is in effect and restrains, enjoins or otherwise prohibits completion of the merger or the other transactions contemplated by the merger agreement. In addition, it is possible that in connection with the review by the Office of the Minnesota AG, or any other antitrust or similar review undertaken by a governmental authority, the reviewing governmental authority imposes costs or limitations on Tesoro, any of which might have an adverse effect on Tesoro following the merger.

Tesoro and Western Refining will be subject to business uncertainties while the merger is pending, which could adversely affect their respective businesses.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on Tesoro and Western Refining. These uncertainties may impair Tesoro’s and Western Refining’s ability to attract, retain and motivate key personnel until the merger is completed and for a period of time thereafter, and could cause customers and others that deal with Tesoro and Western Refining to seek to change their existing business relationships with Tesoro and Western Refining, respectively. Employee retention at Western Refining may be particularly challenging during the pendency of the merger, as employees may experience uncertainty about their roles with Tesoro following the merger. In addition, the merger agreement restricts Tesoro and Western Refining from entering into certain corporate transactions and taking other specified actions without the consent of the other party, and generally requires Western Refining to continue its operations in the ordinary course, until completion of the merger. These restrictions may prevent Tesoro and Western Refining from pursuing attractive business opportunities that may arise prior to the completion of the merger. Please see the section entitled “ The Merger Agreement—Interim Operations of Western Refining and Tesoro Pending the Merger ,” beginning on page 149, for a description of the restrictive covenants to which Tesoro and Western Refining are subject.

Directors and executive officers of Tesoro may have interests in the merger that are different from, or in addition to, the interests of Tesoro stockholders.

Tesoro’s executive officers and directors have interests in the merger that may be different from, or in addition to, the interests of Tesoro stockholders generally. The members of the Tesoro board were aware of and considered these interests, among other matters, when they approved the merger agreement and recommended that Tesoro stockholders approve the Tesoro issuance proposal. These interests are described in more detail in the section entitled “ The Merger—Interests of Tesoro Directors and Executive Officers in the Merger ” beginning on page 132.

Directors and executive officers of Western Refining may have interests in the merger that are different from, or in addition to, the interests of Western Refining stockholders.

Directors and executive officers of Western Refining may have interests in the merger that are different from, or in addition to, the interests of Western Refining stockholders generally. These interests include, among others, the treatment of outstanding equity and equity-based awards pursuant to the merger agreement; potential severance and other benefits upon a qualifying termination in connection with the merger; Tesoro’s agreement to nominate Paul Foster and Jeff Stevens, each of whom currently serves as a director of Western Refining, for election as directors of Tesoro, with terms commencing upon the completion of the merger; and rights to ongoing indemnification and insurance coverage.

These interests are described in more detail in the section entitled “ The Merger—Interests of Western Refining Directors and Executive Officers in the Merger ” beginning on page 133.

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The merger may not be accretive, and may be dilutive, to Tesoro’s earnings per share, which may negatively affect the market price of shares of Tesoro common stock.

Because shares of Tesoro common stock will be issued in the merger, it is possible that, although Tesoro currently expects the merger to be accretive to earnings per share, the merger may be dilutive to Tesoro earnings per share, which could negatively affect the market price of shares of Tesoro common stock.

In connection with the completion of the merger, based on the number of issued and outstanding shares of Western Refining common stock as of February 8, 2017 and the number of outstanding Western Refining equity awards currently estimated to be payable in Tesoro common stock following the merger, and depending on how many Western Refining stockholders make a cash election, Tesoro will issue up to approximately 47,781,800 shares of Tesoro common stock. The issuance of these new shares of Tesoro common stock could have the effect of depressing the market price of shares of Tesoro common stock, through dilution of earnings per share or otherwise. Any dilution of, or delay of any accretion to, Tesoro earnings per share could cause the price of shares of Tesoro common stock to decline or increase at a reduced rate.

Tesoro and Western Refining will incur significant transaction and merger-related costs in connection with the merger, which may be in excess of those anticipated by Tesoro or Western Refining.

Each of Tesoro and Western Refining has incurred and will incur substantial expenses in connection with the negotiation and completion of the transactions contemplated by the merger agreement, including the costs and expenses of filing, printing and mailing this joint proxy statement/prospectus and all filing and other fees paid to the SEC in connection with the merger.

Tesoro and Western Refining expect to continue to incur a number of non-recurring costs associated with completing the merger, combining the operations of the two companies and achieving desired synergies. These fees and costs have been, and will continue to be, substantial. The substantial majority of non-recurring expenses will consist of transaction costs related to the merger and include, among others, employee retention costs, fees paid to financial, legal and accounting advisors, severance and benefit costs and filing fees.

Tesoro and Western Refining will also incur transaction fees and costs related to formulating and implementing integration plans, including facilities and systems consolidation costs and employment-related costs. Tesoro and Western Refining will continue to assess the magnitude of these costs, and additional unanticipated costs may be incurred in the merger and the integration of the two companies’ businesses. Although Tesoro and Western Refining each expects that the elimination of duplicative costs, as well as the realization of other efficiencies related to the integration of the businesses, should allow Tesoro and Western Refining to offset integration-related costs over time, this net benefit may not be achieved in the near term, or at all. See the risk factor entitled “ The integration of Western Refining into Tesoro may not be as successful as anticipated ” below.

The costs described above, as well as other unanticipated costs and expenses, could have a material adverse effect on the financial condition and operating results of Tesoro following the completion of the merger.

Many of these costs will be borne by Tesoro and/or Western Refining even if the merger is not completed.

Lawsuits have been filed against Western Refining and the members of its board of directors challenging the adequacy of the disclosures made in the initial version of the joint proxy statement/prospectus filed on December 14, 2016, and an adverse ruling in one or more of these lawsuits may prevent the merger from being completed.

As of February 8, 2017, Western Refining and the members of its board of directors have been named as defendants in five purported stockholder class actions filed in the United States District Court for the Western District of Texas by Western Refining stockholders challenging the adequacy of the disclosures to Western Refining stockholders made in the initial version of the joint proxy statement/prospectus filed on December 14,

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2016 regarding the transaction. Although Tesoro, Tahoe Merger Sub 1, Inc., and Tahoe Merger Sub 2, LLC are included as defendants in the caption of three of the five complaints, the body of those complaints does not identify those entities as defendants, and the complaints do not appear to assert any claims against them. None of the parties named as defendants in the case captions have been served with the complaint in any of these actions as of the date of this filing. Among other remedies, the plaintiffs seek to enjoin the merger until additional information relating to the transaction is disclosed. The actions also seek to recover costs and disbursements from the defendants, including attorneys’ fees and experts’ fees. The lawsuits are in a preliminary stage.

Additionally, the members of the Tesoro board of directors have been named as defendants in one purported stockholder class action filed in the Court of Chancery of the State of Delaware by a Tesoro stockholder. The suit is captioned Carl Arias v. Gregory J Goff, et al. , Case No. 2017-0094-(filed February 7, 2017). The complaint alleges that Tesoro’s directors breached their fiduciary duties of care, loyalty, good faith and/or disclosure by failing to disclose to Tesoro stockholders all material information necessary to make an informed decision regarding the Tesoro issuance proposal. Among other remedies, the plaintiff seeks to enjoin the merger and to hold Tesoro’s directors liable for allegedly breaching their fiduciary duties.

Additional lawsuits may be filed against Western Refining, Tesoro, Tahoe Merger Sub 1, Inc., Tahoe Merger Sub 2, LLC, and/or the board of directors of any such company in connection with the merger. See the section entitled “ The Merger—Litigation and Regulatory Reviews / Investigations Related to the Merger ” beginning on page 130 of this joint proxy statement/prospectus for more information about litigation related to the merger.

One of the conditions to the closing of the merger is that there must not have been enacted, issued, promulgated, enforced or entered by a court or other governmental entity of competent jurisdiction any applicable law that is in effect and restrains, enjoins or otherwise prohibits completion of the merger or the other transactions contemplated by the merger agreement. Therefore, if any of the plaintiffs in one or more of these actions or in any additional lawsuits that may be filed secures injunctive relief or other relief prohibiting, delaying or otherwise adversely affecting the defendants’ ability to complete the merger, then such injunctive or other relief may prevent the merger from becoming effective within the expected timeframe or at all.

The opinions of Tesoro’s and Western Refining’s respective financial advisors will not reflect changes in circumstances between the signing of the merger agreement and the completion of the merger.

Tesoro and Western Refining have received opinions from their respective financial advisors in connection with the signing of the merger agreement, but have not obtained updated opinions from their respective financial advisors as of the date of this joint proxy statement/prospectus. Changes in the operations and prospects of Tesoro or Western Refining, general market and economic conditions and other factors that may be beyond the control of Tesoro or Western Refining, and on which Tesoro’s and Western Refining’s financial advisors’ opinions were based, may significantly alter the value of Tesoro or Western Refining or the prices of the shares of Tesoro common stock or of the shares of Western Refining common stock by the time the merger is completed. The opinions do not speak as of the time the merger will be completed or as of any date other than the date of such opinions. Because Tesoro and Western Refining do not currently anticipate asking their respective financial advisors to update their opinions, the opinions will not address the fairness of the per share merger consideration from a financial point of view at the time the merger is completed. The Tesoro board’s recommendation that Tesoro stockholders vote “ FOR ” approval of the Tesoro issuance proposal and the other merger-related matters and the Western Refining board’s recommendation that Western Refining stockholders vote “ FOR ” adoption of the merger agreement and the other merger-related matters, however, is made as of the date of this joint proxy statement/prospectus. For a description of the opinions that Tesoro and Western Refining received from their respective financial advisors, please see the sections entitled “ The Merger—Opinion of Goldman Sachs, Tesoro’s Financial Advisor ” and “ The Merger—Opinion of Barclays Capital, Western Refining’s Financial Advisor ” beginning on pages 84 and 97, respectively. A copy of the opinion of Barclays, Western Refining’s financial advisor, is attached as Annex E to this joint proxy statement/prospectus, and a copy

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of the opinion of Goldman Sachs, Tesoro’s financial advisor, is attached as Annex F to this joint proxy statement/prospectus, and each is incorporated by reference herein in its entirety.

If the merger is completed, Tesoro will incur a substantial amount of debt.

In connection with the merger, Tesoro will incur approximately $2.15 billion of additional indebtedness in the aggregate to fund the cash consideration payable under the merger agreement, redeem or repay in full, as applicable, certain outstanding debt of Western Refining and certain of its subsidiaries and pay fees and expenses associated with the foregoing. In addition, following completion of the merger, TLLP will have approximately $3.77 billion aggregate principal amount of senior notes outstanding in addition to any amounts outstanding under its credit facilities, and MLP will have approximately $300 million aggregate principal amount of senior notes outstanding in addition to any amounts outstanding under its revolving credit facility. TLLP’s debt is non-recourse to Tesoro, except for Tesoro Logistics GP, LLC. MLP’s debt is non-recourse to Western Refining. Following the completion of the merger, the combined company will have a significant amount of outstanding indebtedness. The consolidated indebtedness of Tesoro as of September 30, 2016 was approximately $4.7 billion. Tesoro’s pro forma indebtedness as of September 30, 2016, after giving effect to the merger, would be approximately $7.6 billion. This level of indebtedness could have the effect, among other things, of reducing Tesoro’s flexibility to respond to changing business and economic conditions and increasing Tesoro’s interest expense. The increased levels of indebtedness following completion of the merger could also reduce funds available for Tesoro’s investments in capital expenditures, share repurchases, dividend payments and other activities and may create competitive disadvantages for Tesoro relative to competitors with lower debt levels.

In addition, in connection with executing Tesoro’s business strategies following the merger, Tesoro expects to continue to evaluate the possibility of acquiring additional assets and making further strategic investments, and Tesoro may elect to finance these endeavors by incurring additional indebtedness. Moreover, MLP’s notes will remain outstanding following the completion of the merger as described above. In the event that the rating of MLP’s notes are downgraded within 90 days of completion of the merger, a change of control may be triggered with respect to MLP’s notes, and Tesoro may seek to incur additional debt in order to make an offer to repurchase or redeem all such notes.

Tesoro’s ability to arrange any additional financing for the purposes described above or otherwise will depend on, among other factors, the company’s financial position and performance, as well as prevailing market conditions and other factors beyond Tesoro’s control. Tesoro cannot assure you that it will be able to obtain such financing on terms acceptable to Tesoro or at all.

The unaudited pro forma condensed combined financial information and unaudited forecasted financial information included in this joint proxy statement/prospectus is presented for illustrative purposes only and does not represent the actual financial position or results of operations of the combined company following the completion of the merger. Future results of Tesoro or Western Refining may differ, possibly materially, from the unaudited pro forma condensed combined financial information and unaudited forecasted financial information presented in this joint proxy statement/prospectus.

The unaudited pro forma condensed combined financial statements and unaudited forecasted financial information contained in this joint proxy statement/prospectus is presented for illustrative purposes only, contains a variety of adjustments, assumptions and preliminary estimates and does not represent the actual financial position or results of operations of Tesoro and Western Refining prior to the merger or that of the combined company following the merger for several reasons. Specifically, the unaudited pro forma condensed combined financial statements do not reflect the effect of any potential divestitures that may occur prior to or subsequent to the completion of the merger, integration costs or Tesoro’s projected reduction of its debt to capitalization ratio following the completion of the merger. See the sections entitled “Unaudited Pro Forma Condensed Combined Financial Statements ”, and “ Notes To Unaudited Pro Forma Financial Statements ” beginning on pages 177 and 182, respectively. In addition, the merger and post-merger integration process may give rise to unexpected liabilities and costs, including costs associated with the defense and resolution of transaction-related litigation or

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other claims. Unexpected delays in completing the merger or in connection with the post-merger integration process may significantly increase the related costs and expenses incurred by Tesoro. The actual financial positions and results of operations of Tesoro and Western Refining prior to the merger and that of the combined company following the merger may be different, possibly materially, from the unaudited pro forma condensed combined financial statements or forecasted financial information included in this joint proxy statement/prospectus. In addition, the assumptions used in preparing the unaudited pro forma condensed combined financial statements and forecasted financial information included in this joint proxy statement/prospectus may not prove to be accurate and may be affected by other factors. Any significant changes in the market price of Tesoro common stock may cause a significant change in the purchase price used for Tesoro’s accounting purposes and the unaudited pro forma financial statements contained in this joint proxy statement/prospectus.

The effective tax rate that will apply to Tesoro is uncertain and may vary from expectations.

There can be no assurance that the merger will allow Tesoro to maintain any particular effective corporate tax rate. No assurances can be given as to what Tesoro’s effective tax rate will be after the completion of the merger because of, among other things, uncertainty regarding the tax policies of the jurisdictions where it operates. Tesoro’s actual effective tax rate may vary from its expectations and that variance may be material.

Changes in the rules governing U.S. federal income taxation may have an adverse effect on the combined company after the merger.

Policy positions taken by the new presidential administration in the United States may result in significant changes in the rules governing U.S. federal income taxation, including changes to the tax rates, the ability to take certain deductions and/or the ability to pass through income in certain circumstances. Any such changes may have an adverse effect on Tesoro’s or Western Refining’s business, financial condition and results of operations.

The integration of Western Refining into Tesoro may not be as successful as anticipated.

The merger involves numerous operational, strategic, financial, accounting, legal, tax and other risks; potential liabilities associated with the acquired businesses; and uncertainties related to design, operation and integration of Western Refining’s internal control over financial reporting. Difficulties in integrating Western Refining into Tesoro may result in Western Refining performing differently than expected, in operational challenges or in the failure to realize anticipated expense-related efficiencies. Tesoro’s and Western Refining’s existing businesses could also be negatively impacted by the merger. Potential difficulties that may be encountered in the integration process include, among other factors:

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Tesoro’s results may suffer if it does not effectively manage its expanded operations following the merger.

Following completion of the merger, Tesoro’s success will depend, in part, on its ability to manage its expansion, which poses numerous risks and uncertainties, including the need to integrate the operations and business of Western Refining into its existing business in an efficient and timely manner, to combine systems and management controls and to integrate relationships with customers, vendors and business partners.

Even if Tesoro and Western Refining complete the merger, Tesoro may fail to realize all of the anticipated benefits of the proposed merger.

The success of the proposed merger will depend, in part, on Tesoro’s ability to realize the anticipated benefits and cost savings from combining Tesoro’s and Western Refining’s businesses, including the $350 to $425 million in annual pre-tax operational, commercial and corporate synergies that Tesoro believes the combined company will achieve within the first two years following the consummation of the merger. The anticipated benefits and cost savings of the proposed merger may not be realized fully or at all, or may take longer to realize than expected or could have other adverse effects that Tesoro does not currently foresee. Some of the assumptions that Tesoro has made, such as the achievement of operating synergies and the expansion in opportunities for logistics growth in crude oil production basins and regions, may not be realized. The integration process may, for each of Tesoro and Western Refining, result in the loss of key employees, the disruption of ongoing businesses or inconsistencies in standards, controls, procedures and policies. There could be potential unknown liabilities and unforeseen expenses associated with the merger that were not discovered in the course of performing due diligence.

The market price of shares of Tesoro common stock may decline in the future as a result of the sale of shares of Tesoro common stock held by former Western Refining stockholders or current Tesoro stockholders.

Based on the number of shares of Western Refining common stock outstanding as of February 8, 2017 and the number of outstanding Western Refining equity awards currently estimated to be payable in Tesoro common stock following the merger, Tesoro expects to issue up to approximately 47,781,800 shares of Tesoro common stock to Western Refining stockholders in the merger, depending on how many Western Refining stockholders make a cash election. Following their receipt of shares of Tesoro common stock as stock consideration in the merger, former Western Refining stockholders may seek to sell the shares of Tesoro common stock delivered to them. Other Tesoro stockholders may also seek to sell shares of Tesoro common stock held by them following, or in anticipation of, completion of the merger. These sales (or the perception that these sales may occur), coupled with the increase in the outstanding number of shares of Tesoro common stock, may affect the market for, and the market price of, Tesoro common stock in an adverse manner.

The combined company will record goodwill and other intangible assets that could become impaired and result in material non-cash charges to the results of operations of the combined company in the future.

The merger will be accounted for as an acquisition by Tesoro in accordance with accounting principles generally accepted in the United States. Under the acquisition method of accounting, the assets and liabilities of Western Refining and its subsidiaries will be recorded, as of completion, at their respective fair values and added to those of Tesoro. The reported financial condition and results of operations of Tesoro for periods after completion of the merger will reflect Western Refining balances and results after completion of the merger, but will not be restated retroactively to reflect the historical financial position or results of operations of Western Refining and its subsidiaries for periods prior to the merger. See the section entitled “ Unaudited Pro Forma Condensed Combined Financial Statements ” beginning on page 177.

Under the acquisition method of accounting, the total purchase price will be allocated to Western Refining’s tangible assets and liabilities and identifiable intangible assets based on their fair values as of the date of completion of the merger. The excess of the purchase price over those fair values will be recorded as goodwill. Tesoro and Western Refining expect that the merger will result in the creation of goodwill based upon the application of the acquisition method of accounting. To the extent the value of goodwill or intangibles becomes

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impaired, the combined company may be required to incur material non-cash charges relating to such impairment. The combined company’s operating results may be significantly impacted from both the impairment and the underlying trends in the business that triggered the impairment.

Risks Relating to Tesoro’s Business.

You should read and consider risk factors specific to Tesoro’s businesses that will also affect the combined company after the completion of the merger. These risks are described in Part I, Item 1A of Tesoro’s Annual Report on Form 10-K for the year ended December 31, 2015, and in other documents that are incorporated by reference herein. See the section entitled “ Where You Can Find More Information ” beginning on page 211 for the location of information incorporated by reference in this joint proxy statement/prospectus.

Risks Relating to Western Refining’s Business.

You should read and consider risk factors specific to Western Refining’s businesses that will also affect the combined company after the completion of the merger. These risks are described in Part I, Item 1A of Western Refining’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015, and in other documents that are incorporated by reference herein. See the section entitled “ Where You Can Find More Information ” beginning on page 211 for the location of information incorporated by reference in this joint proxy statement/prospectus.

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INFORMATION ABOUT THE COMPANIES

Tesoro Corporation

19100 Ridgewood Parkway

San Antonio, TX 78259

Phone: 210-626-6000

Tesoro, whose legal name is Tesoro Corporation, was incorporated in Delaware in 1968. Based in San Antonio, Texas, Tesoro is one of the largest independent petroleum refining, logistics and marketing companies in the United States. Tesoro’s subsidiaries, operating through three business segments, primarily transport crude oil and manufacture, transport and sell transportation fuels. Tesoro’s refining operating segment refines crude oil and other feedstocks into transportation fuels, such as gasoline and gasoline blendstocks, jet fuel and diesel fuel, as well as other products, including heavy fuel oils, liquefied petroleum gas and petroleum coke for sale in bulk markets to a wide variety of customers within Tesoro’s markets. Tesoro’s logistics operating segment, which is comprised of the assets and operations of TLLP, a publicly traded limited partnership, includes certain crude oil and natural gas gathering assets, natural gas and natural gas liquid processing assets, and crude oil and refined products terminalling, transportation and storage assets acquired from Tesoro and third parties. Tesoro’s marketing operating segment sells transportation fuels through branded and unbranded channels. The branded business sells transportation fuels using a unique brand portfolio with the ARCO ^® , Shell ^® , Exxon ^® , Mobil ^® , USA Gasoline ^TM , Rebel ^TM , Thrifty ^TM and Tesoro ^® brands across a network of over 2,400 retail stations. Tesoro owns 100% of the outstanding equity interests of Tesoro Logistics GP, LLC, the general partner of TLLP. Additionally, as of December 31, 2016, Tesoro owned approximately 33% of the outstanding common units of TLLP.

Western Refining

212 North Clark Drive

El Paso, Texas 79905

Phone: 915-775-3300

Western Refining, whose legal name is Western Refining, Inc., is an independent crude oil refiner and marketer of refined products incorporated in September 2005 under Delaware law with principal offices located in El Paso, Texas. Western Refining produces refined products at refineries in El Paso, Texas, near Gallup, New Mexico and in St. Paul Park, Minnesota. Western Refining sells refined products primarily in Arizona, Colorado, Minnesota, New Mexico, Wisconsin, West Texas, the Mid-Atlantic region and Mexico through bulk distribution terminals and wholesale marketing networks. Western Refining owns a significant limited partner interest in MLP, a publicly traded limited partnership. MLP owns and operates logistics assets consisting of pipeline and gathering, terminalling, storage and transportation assets as well as a wholesale business that operates primarily in the Southwest. MLP operates its logistics assets primarily for the benefit of Western Refining. Western Refining also sells refined products through two retail networks with a total of 545 company-operated and franchised retail sites in the United States. Western Refining owns 100% of the outstanding equity interests of Western Refining Logistics GP, LLC, the general partner of MLP. Additionally, as of December 31, 2016, Western Refining owned approximately 52.6% of the outstanding common units of MLP.

Tahoe Merger Sub 1, Inc.

c/o Tesoro Corporation

19100 Ridgewood Parkway

San Antonio, TX 78259

Phone: 210-626-6000

Merger Sub 1, whose legal name is Tahoe Merger Sub 1, Inc., is a direct, wholly owned subsidiary of Tesoro. Upon the completion of the merger, Merger Sub 1 will cease to exist. Merger Sub 1 was incorporated in Delaware on November 15, 2016 for the sole purpose of effecting the merger.

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Tahoe Merger Sub 2, LLC

c/o Tesoro Corporation

19100 Ridgewood Parkway

San Antonio, TX 78259

Phone: 210-626-6000

Merger Sub 2, whose legal name is Tahoe Merger Sub 2, LLC, is a direct, wholly owned subsidiary of Tesoro. If a second merger election is made, upon the completion of the second merger, Merger Sub 2 will survive such second merger and continue to exist as a direct, wholly owned subsidiary of Tesoro. Merger Sub 2 was formed in Delaware on November 15, 2016 for the sole purpose of effecting the second merger (if a second merger election is made).

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SPECIAL MEETING OF TESORO STOCKHOLDERS

Date, Time and Place

The Tesoro special meeting will be held on March 24, 2017, at 12:00 p.m., Central Time, at 19100 Ridgewood Parkway, San Antonio, Texas 78259.

Purpose of the Tesoro Special Meeting

The purpose of the Tesoro special meeting is as follows:

Tesoro will transact no other business at the Tesoro special meeting.

Recommendation of Tesoro’s Board of Directors

The Tesoro board recommends that Tesoro stockholders vote:

1. “ FOR ” the approval of the Tesoro issuance proposal;

2. “ FOR ” the approval of the Tesoro certificate of incorporation amendment proposal; and

3. “ FOR ” the approval of the Tesoro adjournment proposal.

See the section entitled “ The Merger—Recommendation of Tesoro’s Board of Directors and Reasons for the Merger ” beginning on page 81.

Record Date

Only holders of record of issued and outstanding shares of Tesoro common stock as of the close of business on February 10, 2017, the record date for the Tesoro special meeting, are entitled to notice of, and to vote at, the Tesoro special meeting or any adjournment or postponement of the Tesoro special meeting.

Quorum; Required Votes; Abstentions and Broker Non-Votes

A quorum of Tesoro stockholders is necessary to hold a valid meeting. A quorum will exist at the Tesoro special meeting with respect to each matter to be considered at the Tesoro special meeting if the holders of a majority of shares of Tesoro common stock outstanding on the record date are present in person or represented by proxy at the Tesoro special meeting. All shares represented by proxy are counted as present for purposes of establishing a quorum, including abstentions and broker non-votes. A broker non-vote occurs when a broker submits a proxy that states that the broker does not vote for some or all of the proposals because the broker has not received instructions from the beneficial owners on how to vote on the proposals and does not have discretionary authority to vote in the absence of instructions. If a quorum is not established with respect to a particular subject matter, the presiding person of the Tesoro special meeting has the power to adjourn or recess the Tesoro special meeting from time to time until a quorum is present with respect to that subject matter.

If you submit a properly executed proxy card, even if you do not vote for some or all of the proposals or vote to “abstain” in respect of some or all of the proposals, your shares of Tesoro common stock will be counted for purposes of calculating whether a quorum is present at the Tesoro special meeting with respect to each matter

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to be considered at the Tesoro special meeting. Executed but unvoted proxies will be voted in accordance with the recommendations of the Tesoro board. If additional votes must be solicited to approve the Tesoro issuance proposal, it is expected that the meeting will be adjourned to solicit additional proxies.

Approval of the Tesoro issuance proposal requires the affirmative vote of a majority of votes cast by Tesoro stockholders present in person or by proxy at the Tesoro special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

Approval of the Tesoro certificate of incorporation amendment proposal requires the affirmative vote of a majority of the shares of Tesoro common stock outstanding as of the close of business on the record date and entitled to vote on the proposal. Abstentions and broker non-votes will have the same effect as a vote “AGAINST” the proposal.

Approval of the Tesoro adjournment proposal requires the affirmative vote of a majority of the shares of Tesoro common stock present in person or by proxy at the Tesoro special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

The matters to be voted on at the Tesoro special meeting are described in the section entitled “Tesoro Proposals” beginning on page 62.

Methods of Voting

Registered stockholders, whether holding shares directly as stockholders of record or beneficially in “street name,” may vote via the Internet by going to the web address provided on the enclosed proxy card and following the instructions for Internet voting; by telephone using the toll-free telephone number listed on the enclosed proxy card; or by completing, signing, dating and returning the enclosed proxy card in the postage-paid envelope provided.

Stockholders of record may vote their shares (other than shares held in the Tesoro Corporation Thrift Plan) in person by ballot at the Tesoro special meeting or by submitting their proxies:

If your shares are held in the Tesoro Corporation Thrift Plan, you many not vote in person at the Tesoro special meeting. Instead you will need to vote by submitting your proxy:

Stockholders of Tesoro who hold their shares in “street name” by a broker, nominee, fiduciary or other custodian should refer to the proxy card or other information forwarded by their broker, nominee, fiduciary or other custodian for instructions on how to vote their shares.

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Voting in Person

Shares held directly in your name as stockholder of record may be voted in person at the Tesoro special meeting. If you choose to vote your shares in person at the Tesoro special meeting, please bring your enclosed proxy card and proof of identification. Even if you plan to attend the Tesoro special meeting, the Tesoro board recommends that you vote your shares in advance as described below so that your vote will be counted if you later decide not to attend the Tesoro special meeting. Please note that if your shares are held in the Tesoro Corporation Thrift Plan, you may not vote in person at the Tesoro special meeting, instead you will need to submit your vote through one of the ways described above.

If you are a beneficial holder, you will receive separate voting instructions from your broker, bank or other nominee explaining how to vote your shares. Please note that if your shares are held in “street name” by a broker, bank or other nominee and you wish to vote at the Tesoro special meeting, you will not be permitted to vote in person unless you first obtain a legal proxy issued in your name from the record owner. You are encouraged to request a legal proxy from your broker, bank or other nominee promptly as the process can be lengthy.

Voting by Proxy

Whether you hold your shares directly as the stockholder of record or beneficially in “street name,” you may direct your vote by proxy without attending the Tesoro special meeting. You can vote by proxy via the Internet, by telephone or by mail by following the instructions provided in the enclosed proxy card.

Questions About Voting

If you have any questions about how to vote or direct a vote in respect of your shares of Tesoro common stock, you may contact Innisfree, Tesoro’s proxy solicitor, at:

Shareholders may call toll-free at 1-888-750-5834.

Banks and brokers may call collect at 1-212-750-5833.

Revocability of Proxies

If you are a stockholder of record of Tesoro, whether you vote by telephone, Internet or mail, you can change or revoke your proxy before it is voted at the meeting in one of the following ways: