UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

SOTHEBY’S

(Name of Issuer)

Common Stock, par value $0.01 per share

(Title of Class of Securities)

835898107

(CUSIP Number)

Neil S. Bhatia, Esq.

Marcato Capital Management LP

One Montgomery Street, Suite 3250

San Francisco, CA 94104

(415) 796-6350

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

COPIES TO:

Jeffrey L. Kochian, Esq.

Akin Gump Strauss Hauer & Feld LLP

One Bryant Park

New York, NY 10036

(212) 872-1000

February 20, 2015

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box  ¨.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 835898107

CUSIP No. 835898107

CUSIP No. 835898107

CUSIP No. 835898107

CUSIP No. 835898107

CUSIP No. 835898107

SCHEDULE 13D

This Amendment No. 6 supplements the information set forth in the Schedule 13D filed by the Reporting Persons with the United States Securities and Exchange Commission (the “SEC”) on July 30, 2013, as amended from time to time (the “Schedule 13D”), relating to Common Stock, par value $0.01 per share (the “Shares”), of Sotheby’s, a Delaware corporation (the “Issuer”). All capitalized terms contained herein but not otherwise defined shall have the meanings ascribed to such terms in the Schedule 13D.

The information set forth in response to each separate Item below shall be deemed to be a response to all Items where such information is relevant. The Schedule 13D is hereby supplementally amended as follows:

The responses to Items 4, 5 and 6 of the Schedule 13D are incorporated herein by reference.

The net investment costs (including commissions, if any) of the Shares directly owned by the private investment funds advised by Marcato is approximately $204,025,017.

The responses to Items 3, 5 and 6 of the Schedule 13D are incorporated herein by reference.

Attached hereto is a letter (the “Letter”) sent by Marcato to Domenico De Sole, Lead Independent Director of the Issuer, in which Marcato details suggestions regarding the Issuer’s capital allocation policies that Marcato believes will enhance shareholder value. The Letter is filed as Exhibit 5 and is incorporated herein by reference.

(a)—(e) As of the date hereof, (i) Marcato and Mr. McGuire may be deemed to be the beneficial owners of 6,571,806 Shares (the “Marcato Shares”) constituting 9.53% of the Shares, (ii) Marcato, L.P. may be deemed to be the beneficial owner of 1,529,210 Shares, constituting 2.22% of the Shares, (iii) Marcato II, L.P. may be deemed to be the beneficial owner of 117,754 Shares, constituting 0.17% of the Shares and (iv) Marcato International Master Fund, Ltd. may be deemed to be the beneficial owner of 4,924,842 Shares, constituting 7.14% of the Shares, each based upon a total of 68,990,927 Shares outstanding as of October 31, 2014 (based on the Issuer’s Quarterly Report on Form 10-Q, filed with the SEC on November 10, 2014).

CUSIP No. 835898107

Marcato, L.P. may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) 1,529,210 Shares. Marcato II, L.P. may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) 117,754 Shares. Marcato International Master Fund, Ltd. may be deemed to have the shared power to vote or direct vote of (and the shared power to dispose or direct the disposition of) 4,924,842 Shares. Marcato, as the investment manager of Marcato, L.P., Marcato II, L.P. and Marcato International Master Fund, Ltd., may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) the Marcato Shares. By virtue of Mr. McGuire’s position as managing partner of Marcato, Mr. McGuire may be deemed to have the shared power to vote or direct the vote of (and the shared power to dispose or direct the disposition of) the Marcato Shares and, therefore, Mr. McGuire may be deemed to be the beneficial owner of the Marcato Shares.

The transactions by the Reporting Persons in the securities of the Issuer during the past sixty days are set forth in Exhibit 2.

The limited partners of (or investors in) each of Marcato, L.P., Marcato II, L.P., and Marcato International Master Fund, Ltd., or their respective subsidiaries or affiliated entities, for which Marcato or its affiliates acts as general partner and/or investment manager have the right to participate in the receipt of dividends from, or proceeds from the sale of, the Shares held for the accounts of their respective funds in accordance with their respective limited partnership interests (or investment percentages) in their respective funds.

The responses to Items 3, 4 and 5 of the Schedule 13D are incorporated herein by reference.

The Reporting Persons purchased American-style call options referencing 1,500,000 Shares with a strike price of $39 which are exercisable through April 17, 2015. The Reporting Persons sold American-style call options referencing 1,500,000 Shares with a strike price of $45 which are exercisable through April 17, 2015. None of the options gives the Reporting Persons direct or indirect voting, investment or dispositive control over any securities of the Issuer or requires the counterparties thereto to acquire, hold, vote or dispose of any securities of the Issuer prior to exercise.

The Reporting Persons sold American-style put options referencing 1,500,000 Shares with a strike price of $33 which are exercisable through April 17, 2015. None of the options gives the Reporting Persons direct or indirect voting, investment or dispositive control over any securities of the Issuer or requires the counterparties thereto to acquire, hold, vote or dispose of any securities of the Issuer prior to exercise.

Except for the arrangements described in this Schedule 13D, to the best knowledge of the Reporting Persons, there are no contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such persons and any other person with respect to any securities of the Issuer, including but not limited to, transfer or voting of any of the securities, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantees of profits, division of profits or loss, or the giving or withholding of proxies.

Exhibit 2 – Transactions in the Shares effected in the past sixty days.

Exhibit 5 – Letter from Marcato Capital Management LP to Domenico De Sole, Lead Independent Director of Sotheby’s.

CUSIP No. 835898107

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

EXHIBIT 2

TRANSACTIONS

The following table sets forth all transactions with respect to Shares or options to purchase or sell Shares effected in the last sixty days by the Reporting Persons on behalf of the Reporting Persons, inclusive of any transactions effected through 4:00 p.m., New York City time, on February 19, 2015. All such transactions were purchases or sales of Shares or purchases or sales of options to purchase or sell Shares effected in the open market, and the table includes commissions paid in per share prices.

Exhibit 5

February 20, 2015

Domenico De Sole

Lead Independent Director

Sotheby’s

1334 York Avenue

New York, New York 10021

Domenico,

I am writing in response to Sotheby’s announcement on February 13^th that “there will be no return of capital to shareholders at this time”.¹

In the 18 months since Marcato’s initial investment in Sotheby’s, we have outlined numerous opportunities both publicly and privately for Sotheby’s to use its capital more intelligently to create shareholder value. Despite our dialogue with you and other members of the board, a substantial portion of Sotheby’s invested capital continues to earn a poor return or worse yet, earns no return at all. This willful neglect on the part of both management and the Finance Committee of the board must end urgently. Shareholders deserve leadership that combines sound business strategy with skilled financial management. For the duration of Marcato’s investment, we have enjoyed neither. We feel a responsibility to other shareholders to express our deep concern with the governance and executive judgment on matters of capital allocation and hold this board and management accountable. We submit that any new CEO’s first order of business may need to include the recruitment of a new CFO who will serve the interests of shareholders rather than defend misguided policies of the past.

By way of background, sell-side analysts covering Sotheby’s value the company by applying a P/E multiple to their estimate of earnings per share. This methodology produces a result that ignores any part of Sotheby’s business that has value but does not contribute to its earnings; the most obvious example of which is the company’s unusually large cash balance, but the same can be said for art and jewelry inventory, excess equity in the loan portfolio, and real estate assets. None of this significant value is being accounted for in Sotheby’s current market value.

Today, we present a specific and straightforward roadmap that we believe can immediately release $12.33 per share (28% of current market value) without materially affecting the company’s continued earnings power. With this increased flexibility, we recommend the immediate repurchase of $500 million of Sotheby’s shares.

How did we get here?

A simple Google search for “Delaware Chancery Court Memorandum Opinion—Sotheby’s” produces a court document that offers rare insight into the rationale behind Sotheby’s current capital allocation strategy. The email excerpts in the court opinion expose that capital allocation decisions in the past had not been governed by thoughtful consideration for liquidity needs, return thresholds, and costs of capital but rather by tactical considerations to forestall a long-overdue proxy contest.

Among numerous other examples, the passage below between CEO Bill Ruprecht and Robert Taubman, a member of the Finance Committee, was particularly disappointing:

“We are going to be the target of a proxy fight with activist shareholders. The motivation for that fight is only peripherally about returning capital. It is about being on Sotheby’s Board. Mick McGuire needs that as validation, and Loeb wants that for ego…

My review of the situation however is this: if we make a gesture, that we can afford, of a couple of hundred million returned quickly to shareholders, we gain enormous tactical leverage in the process of persuading the 85% of shareholders who were not activists, that we are responsible stewards for their investment…

So this is about power, and political gamesmanship with shareholders, not about capital structure… [emphasis added]

If you do a modest buy back, and still feel like you are going to lose a proxy contest, then you would offer activists one or two seats and still be in control.” ²

At the time of this correspondence, Sotheby’s current CFO, Patrick McClymont, was employed by Goldman Sachs and serving as management’s financial advisor. In an email exchange with Bill Ruprecht, Patrick McClymont “agreed” with Bill’s argument that “ the decision to make a prudent distribution now allows us the greatest control over board composition and collegiality going forward. To bunker in ensures board disruption next spring. This isn’t and never was about capital markets [for McGuire and Loeb]. It’s about getting into a leadership position on Sotheby’s board.” [emphasis added]

As you recall, this reasoning proved to be wrong as I was offered a board seat and I declined because I would only serve on the condition that Sotheby’s commit to certain capital allocation targets; a condition the company resisted.

Against this backdrop, should shareholders be surprised that a CFO who was originally hired as the CEO’s activist defense banker would be reluctant to update a capital allocation strategy that was conceived with ulterior motives? Should shareholders be surprised that promises to return capital have gone unfulfilled after the ill-conceived plan failed to accomplish its goal of preventing a proxy challenge? Should shareholders be surprised that a board and management with very little of their own money invested in Sotheby’s stock are unconcerned with the opportunity cost of lazy capital allocation policies?

2

What should we do now?

The exercise is a simple one:

Management has provided a framework to #1 in a presentation dated January 29, 2014 where Sotheby’s detailed the results of its capital allocation and financial policy review³(this presentation mysteriously can no longer be found on Sotheby’s website).

The answer to #1 is $495 million which is broken down as follows:

To reiterate, in January 2014, as part of an effort to undermine a proxy contest through a “gesture we can afford,” management’s framework implies $495 million of worst case scenario liquidity requirements. It is difficult to imagine why management would have any reason to underestimate this number, but quite easy to imagine why they might have wanted to overestimate the number for purposes of justifying a smaller “gesture.”

3

Calculating the sources of capital at Sotheby’s disposal is also not difficult. However, we do believe that there are steps that Sotheby’s can and should take to increase the total liquidity available to the company. We detail this below:

Current Ready Liquidity:

Subtotal: $976 million

We continue to recommend that the company take the following easy steps to create additional sources of liquidity:

Subtotal: $370 million

#2: Total sources of capital: $1,346 million

#2 minus #1 = $851 million of excess capital!

Calculating the Cost:

$851 million of excess capital represents $12.33 per share, or 28% of the current stock price. We believe the company can immediately release this excess capital without materially changing the earnings power of the business.

4

This degree of financial mismanagement is unacceptable particularly given the persistence of excess capital over the tenure of Marcato’s investment. The board and management must take the necessary steps to cure this dynamic. We recommend the immediate repurchase of $500 million of Sotheby’s shares. This amount represents the current cash and revolver capacity less the entire reserve for worst case scenario liquidity requirements. Share repurchases are much more tax efficient than a special dividend – and we believe that Sotheby’s shares are a compelling long-term investment. The remaining excess capital can be accessed in short order and redeployed into activities that clear the company’s 15% ROIC hurdle or can be returned to shareholders through additional share repurchases. Future free cash flow generated by the business will also be available to fund growth investments.

We expect that with this analysis, the Finance Committee of the board will be equipped to fulfill its duty as fiduciaries to shareholders. While these numbers appear large, their magnitude only serves to highlight the dereliction of responsibility of Sotheby’s board and management. As one of Sotheby’s largest shareholders, we also look forward to a thorough discussion with its new CEO regarding the importance of intelligent capital allocation in creating value for shareholders.

Sincerely,

Mick McGuire

cc:

John Angelo – Finance Committee

Jessica Bibliowicz

Kevin Conroy

The Duke of Devonshire

Daniel Loeb

Daniel Meyer

Olivier Reza – Finance Committee

Marsha Simms

Robert Taubman – Finance Committee

Diana Taylor – Finance Committee

Dennis Weibling – Finance Committee, Chairman

Harry Wilson – Finance Committee

5