Common Stock, par value $0.01 per share Underwriting Agreement

Exhibit 1.1

Execution Copy

Dean Foods Company

22,500,000 Shares ¹

Common Stock, par value $0.01 per share

Underwriting Agreement

May 5, 2009

MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED

J.P. MORGAN SECURITIES INC.

BARCLAYS CAPITAL INC.

WACHOVIA CAPITAL MARKETS, LLC

    As Representatives of the several Underwriters

c/o Merrill Lynch, Pierce, Fenner & Smith Incorporated

One Bryant Park New York, New York 10036

Ladies and Gentlemen:

Dean Foods Company, a corporation organized under the laws of the State of Delaware (the “Company”), proposes to issue and sell an aggregate of 22,500,000 shares (the “Firm Stock”) of the Company’s common stock, par value, $0.01 per share (the “Common Stock”) to the several underwriters named in Schedule I (the “Underwriters”) pursuant to this agreement. In addition, the Company has granted to the Underwriters an option to purchase up to an additional 3,375,000 shares (the “Optional Stock”) of Common Stock to cover overallotments, if any, as provided in Section 2 hereof. The Firm Stock and, if and to the extent such option is exercised, the Optional Stock are collectively called the “Stock”. Merrill Lynch, Pierce, Fenner & Smith Incorporated (“Merrill Lynch”), J.P. Morgan Securities Inc. (“J.P. Morgan”), Barclays Capital Inc. and Wachovia Capital Markets, LLC have agreed to act as representatives of the several Underwriters (in such capacity, the “Representatives”) in connection with the offering and sale of the Stock. This is to confirm the agreement concerning the purchase of the Stock from the Company by the Underwriters. Any reference herein to the Registration Statement, the Base Prospectus, any preliminary prospectus or the Prospectus shall be deemed to refer to and include the documents incorporated by reference therein pursuant to Item 12 of Form S-3 which were filed under the Exchange Act on or before the Effective Date of the Registration Statement or the date of the Base Prospectus, such preliminary prospectus or the Prospectus, as the case may be; and any reference herein to the terms “amend,” “amendment” or “supplement” with respect to the Registration Statement, the Base Prospectus, any preliminary prospectus or the Prospectus shall be deemed to refer to and include the filing of any document under the Exchange Act after the Effective Date of the Registration Statement or the date of the Base Prospectus, such preliminary prospectus or the Prospectus, as the case may be, deemed to be incorporated therein by reference. Certain terms used herein are defined in Section 20 hereof.

1. Representations and Warranties . The Company represents and warrants to, and agrees with, each Underwriter as set forth below in this Section 1.

(a) The Company meets the requirements for use of Form S-3 under the Act and has prepared and filed with the Commission an “automatic shelf registration statement” (as defined in Rule 405) (File number 333-149439) on Form S-3, including a related Base Prospectus, for registration under the Act of the offering and sale of the Stock. The Registration Statement initially became effective within three years of the date hereof. No notice of objection of the Commission to the use of the Registration Statement or any post-effective amendment thereto pursuant to Rule 401(g)(2) has been received by the Company. No order suspending the effectiveness of the Registration Statement, the Base Prospectus, any preliminary prospectus or the Prospectus has been issued by the Commission and no proceeding for that purpose or pursuant to Section 8A of the Securities Act against the Company or related to the offering has been initiated or threatened by the Commission. Such Registration Statement, including any amendments thereto filed prior to the Applicable Time, became effective upon filing. The Company will file with the Commission the Prospectus relating to the Stock in accordance with Rule 424(b). As filed, the Prospectus shall contain all information required by the Act and the rules thereunder, and, except to the extent the Representatives shall agree in writing to a modification, shall be in all substantive respects in the form furnished to you prior to the Applicable Time or, to the extent not completed at the Applicable Time, shall contain only such specific additional information and other changes (beyond that contained in the Base Prospectus) as the Company has advised you, prior to the Applicable Time, will be included or made therein. The Registration Statement, at the Applicable Time, meets the requirements set forth in Rule 415(a)(1)(x).

(b) On each Effective Date and at the Applicable Time, the Registration Statement did and will, and each preliminary prospectus and the Prospectus when first filed in accordance with Rule 424(b) and on the Closing Date and any Subsequent Closing Date, each preliminary prospectus and the Prospectus (and any supplement thereto) will, comply in all material respects with the applicable requirements of the Act, the Exchange Act and the respective rules thereunder; on each Effective Date and at the Applicable Time, the Registration Statement did not and will not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading; and on the date of any filing pursuant to Rule 424(b) and on the Closing Date and any Subsequent Closing Date, the Prospectus (together with any supplement thereto) will not include any untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided , however , that the Company makes no representations or warranties as to the information contained in or omitted from the Registration Statement or the Prospectus (or any supplement thereto) in reliance upon and in conformity with information furnished in writing to the Company by or on behalf the Underwriters through the Representatives specifically for inclusion in the Registration Statement or the

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Prospectus (or any supplement thereto), it being understood and agreed that the only such information furnished by or on behalf of the Underwriters consists of the information described as such in Section 8 hereof.

(c) The Disclosure Package does not include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading. The preceding sentence does not apply to statements in or omissions from the Disclosure Package based upon and in conformity with written information furnished to the Company by the Underwriters through the Representatives specifically for use therein, it being understood and agreed that the only such information furnished by or on behalf of the Underwriters consists of the information described as such in Section 8 hereof.

(d) The documents incorporated by reference in the Disclosure Package, the Prospectus and the Registration Statement, when they became effective or were filed with the Commission, as the case may be, conformed in all material respects to the requirements of the Act or the Exchange Act, as applicable. Any further documents so filed and incorporated by reference in the Disclosure Package, the Prospectus and the Registration Statement or any further amendment or supplement thereto, when such documents become effective or are filed with the Commission, as the case may be, will conform in all material respects to the requirements of the Act or the Exchange Act, as applicable, and the rules and regulations of the Commission thereunder.

(e) (i) At the time of filing the Registration Statement, (ii) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the Act (whether such amendment was by post-effective amendment, incorporated report filed pursuant to Sections 13 or 15(d) of the Exchange Act or form of prospectus), (iii) at the time the Company or any person acting on its behalf (within the meaning, for this clause only, of Rule 163(c)) made any offer relating to the Stock in reliance on the exemption in Rule 163, and (iv) at the Applicable Time (with such date being used as the determination date for purposes of this clause (iv)), the Company was or is (as the case may be) a “well-known seasoned issuer” as defined in Rule 405.

(f) (i) At the earliest time after the filing of the Registration Statement that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2)) of the Stock and (ii) as of the Applicable Time (with such date being used as the determination date for purposes of this clause (ii)), the Company was not and is not an Ineligible Issuer (as defined in Rule 405), without taking account of any determination by the Commission pursuant to Rule 405 that it is not necessary that the Company be considered an Ineligible Issuer.

(g) Each Issuer Free Writing Prospectus conformed or will conform in all material respects to the requirements of the Securities Act on the date of first use, and the Company has complied with any filing requirements applicable to such Issuer Free Writing Prospectus under the Securities Act. The Company has not made any offer relating to the Stock that would constitute an Issuer Free Writing Prospectus without the prior written consent of Merrill Lynch and J.P. Morgan. The Company has retained in

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accordance with the requirements of the Securities Act all Issuer Free Writing Prospectuses that were not required to be filed pursuant to the Securities Act. Each Issuer Free Writing Prospectus, when considered together with the Disclosure Package as of the Applicable Time, did not include an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading.

(h) Each of the Company and its subsidiaries (as defined in Section 20) has been duly organized and is validly existing as a corporation (or such other form of legal entity as its name and organizational documents may indicate) in good standing under the laws of the jurisdiction in which it is chartered or organized with full corporate (or other) power and authority to own or lease, as the case may be, and to operate its properties and conduct its business as described in the Disclosure Package and the Prospectus, and, except where it would not have a Material Adverse Effect, is duly qualified to do business as a foreign corporation (or such other form of legal entity as its name and organizational documents may indicate) and is in good standing under the laws of each jurisdiction which requires such qualification.

(i) All the outstanding shares of capital stock or other ownership interests of each subsidiary of the Company have been duly and validly authorized and issued and are fully paid and non-assessable, and, except as otherwise set forth in the Disclosure Package and the Prospectus, all outstanding shares of capital stock or other ownership interests of such subsidiaries are owned by the Company either directly or through wholly-owned subsidiaries free and clear of any perfected security interest or any other security interests, claims, liens or encumbrances, except where such security interests, claims, liens or encumbrances would not have a Material Adverse Effect.

(j) There is no franchise, contract or other document of a character required to be described in the Registration Statement or Prospectus, or to be filed as an exhibit thereto, which is not described or filed as required.

(k) This Agreement has been duly authorized, executed and delivered by the Company.

(l) The shares of the Stock to be issued and sold by the Company to the Underwriters hereunder have been duly authorized and, upon payment and delivery in accordance with this Agreement, will be validly issued, fully paid and non-assessable, will conform to the description thereof contained in the Base Prospectus, will be issued in compliance with federal and state securities laws and will be free of statutory and contractual preemptive rights, rights of first refusal and similar rights.

(m) The statements set forth in the Disclosure Package and the Prospectus under the caption “Description of Capital Stock” are accurate and fair in all material respects.

(n) Neither the Company nor any subsidiary is, or, after giving effect to the offering and sale of the Stock and the application of the proceeds thereof as described in the Disclosure Package and the Prospectus, will be an “investment company” as defined in the Investment Company Act of 1940, as amended.

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(o) No consent, approval, authorization, filing with or order of any court or governmental agency or body is required in connection with the transactions contemplated herein, except such as have been obtained under the Act and the Exchange Act and such as may be required under the blue sky laws of any jurisdiction in connection with the purchase and distribution of the Stock by the Underwriters in the manner contemplated herein and in the Disclosure Package and the Prospectus.

(p) Neither the offer, issue or sale of the Stock nor the consummation of any other of the transactions herein contemplated nor the fulfillment of the terms hereof will conflict with, result in a breach or violation of, or imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to (i) the charter or by-laws or similar organizational documents of the Company or any of its subsidiaries, (ii) the terms of any material indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which the Company or any of its subsidiaries is a party or bound or to which its or their property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree applicable to the Company or any of its subsidiaries of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Company or any of its subsidiaries or any of its or their properties, except, with respect to clauses (ii) and (iii) above, where such conflict, breach, violation or imposition would not, individually or in the aggregate, result in a Material Adverse Effect.

(q) No holder of securities of the Company has rights to the registration of such securities under the Registration Statement.

(r) The consolidated financial statements and schedules of the Company and its consolidated subsidiaries included or incorporated by reference in the Base Prospectus, the Prospectus and the Registration Statement present fairly, in all material respects, the consolidated financial condition, results of operations and cash flows of the Company and its consolidated subsidiaries as of the dates and for the periods indicated, comply as to form with the applicable accounting requirements of the Act and have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods involved (except as otherwise noted therein). No other financial statements are required to be included or incorporated by reference in the Registration Statement.

(s) Since the date of the most recent financial statements included or incorporated by reference in the Prospectus (exclusive of any supplement thereto), there has been no material adverse change, or any development involving a prospective material adverse change, in the condition (financial or otherwise), earnings, business or properties of the Company and its subsidiaries, taken as a whole, whether or not arising from transactions in the ordinary course of business or otherwise, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

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(t) No action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Company or its property is pending or, to the best knowledge of the Company, threatened that (i) could reasonably be expected to have a material adverse effect on the performance of this Agreement or the consummation of any of the transactions contemplated hereby or (ii) could reasonably be expected to have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(u) Each of the Company and its subsidiaries owns or leases all such properties as are necessary to the conduct of its operations as presently conducted. The Company and each of its subsidiaries has good and marketable title to all the properties and assets reflected as owned in the financial statements referred to in paragraph 1(r) above, in each case free and clear of any security interests, mortgages, liens, encumbrances, equities, claims and other defects, except such as would not, individually or in the aggregate, result in a Material Adverse Effect, or except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto). The real property, improvements, equipment and personal property held under lease by the Company or any subsidiary are held under valid and enforceable leases, with such exceptions as would not, individually or in the aggregate, result in a Material Adverse Effect or except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(v) Neither the Company nor any of its subsidiaries is in violation or default of (i) any provision of its charter or bylaws or similar organizational documents, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which it is a party or bound or to which its property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Company or such subsidiary or any of its properties, as applicable, except, with respect to (ii) and (iii) above, for such violations and defaults that would not, individually or in the aggregate, result in a Material Adverse Effect.

(w) Deloitte & Touche LLP, who have certified certain financial statements of the Company and its consolidated subsidiaries and delivered their report with respect to the audited consolidated financial statements and schedules included or incorporated by reference in the Base Prospectus and the Prospectus, are independent public accountants with respect to the Company and its consolidated subsidiaries within the meaning of the Act and the applicable published rules and regulations thereunder.

(x) The statistical and market-related data included under the captions included or incorporated by reference in the Disclosure Package and the Prospectus are based on or derived from sources that the Company believes to be reliable and accurate in all material respects.

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(y) Each of the Company and its subsidiaries (i) has filed all foreign, federal, state and local tax returns that are required to be filed or has requested extensions thereof, except where the failure so to file would not have a Material Adverse Effect, and (ii) has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent they are due and payable, except where any such tax, assessment, fine or penalty is currently being contested in good faith or would not have a Material Adverse Effect, and except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(z) There are no transfer taxes or other similar fees or charges under federal law or the laws of any state, or any political subdivision thereof, required to be paid in connection with the execution and delivery of this Agreement or the issuance by the Company or sale by the Company of the Stock.

(aa) No labor problem or dispute with the employees of the Company or any of its subsidiaries exists or is threatened or imminent, and neither the Company nor any of its subsidiaries is aware of any existing or imminent labor disturbance by the employees of any of its or any of its subsidiaries’ principal suppliers, contractors or customers, that could have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(bb) The Company is insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they are engaged.

(cc) No subsidiary of the Company is currently prohibited, directly or indirectly, from paying any dividends to the Company, from making any other distribution on such subsidiary’s capital stock or other ownership interest, from repaying to the Company any loans or advances to such subsidiary from the Company or from transferring any of such subsidiary’s property or assets to the Company or any other subsidiary of the Company, except as described in or contemplated by the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(dd) The Company and its subsidiaries possess all licenses, certificates, permits and other authorizations issued by the appropriate federal, state or foreign regulatory authorities necessary to conduct their respective businesses, and neither the Company nor any subsidiary has received any notice of proceedings relating to the revocation or modification of any such certificate, authorization or permit which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

(ee) The Company and its subsidiaries maintain a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset

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accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. The Company and its subsidiaries’ internal controls over financial reporting are effective, and, except as disclosed in the Disclosure Package and the Prospectus, or in any document incorporated by reference therein, since the end of the Company’s most recent audited fiscal year, there has been (i) no material weakness in the Company’s and its subsidiaries’ internal controls over financial reporting (whether or not remediated) and (ii) no change in the Company’s and its subsidiaries’ internal control over financial reporting that has materially affected, or is reasonably likely to materially affect, such internal controls.

(ff) The Company and its subsidiaries maintain “disclosure controls and procedures” (as such term is defined in Rule 13a-15(e) under the Exchange Act); such disclosure controls and procedures are effective at the reasonable assurance level.

(gg) There is and has been no failure on the part of the Company or any of its directors or executive officers in their capacities as such, to comply in all material respects with any applicable provision of the Sarbanes-Oxley Act of 2002 and the rules and regulations promulgated in connection therewith (the “Sarbanes-Oxley Act”), including Section 402 related to loans and Sections 302 and 906 related to certifications.

(hh) Except as set forth in the Disclosure Package and the Prospectus (exclusive of any supplement thereto), the Company and each of its subsidiaries are (i) in compliance in all material respects with any and all applicable laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“Environmental Laws”), (ii) have received and are in compliance with all permits, licenses or other approvals required of them under applicable Environmental Laws to conduct their respective businesses, and (iii) have not received notice of any actual or potential liability under any Environmental Law, except where such non-compliance with Environmental Laws, failure to receive required permits, licenses or other approvals, or liability would not, individually or in the aggregate, have a Material Adverse Effect. Except as set forth in the Disclosure Package and the Prospectus (exclusive of any supplement thereto), neither the Company nor any of its subsidiaries has been named as a “potentially responsible party” under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended.

(ii) In the ordinary course of its business, the Company periodically reviews the effect of Environmental Laws on the business, operations and properties of the Company and its subsidiaries, in the course of which it identifies and evaluates associated costs and liabilities. On the basis of such review, the Company and its subsidiaries have reasonably concluded that such associated costs and liabilities would not, singly or in the aggregate, have a Material Adverse Effect, except as set forth in or contemplated in the Disclosure Package and the Prospectus (exclusive of any supplement thereto).

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(jj) The minimum funding standard under Section 302 of the Employee Retirement Income Security Act of 1974, as amended, and the regulations and published interpretations thereunder (“ERISA”), has been satisfied by each “pension plan” (as defined in Section 3(2) of ERISA) which has been established or maintained by the Company and/or one or more of its subsidiaries, and the trust forming part of each such plan which is intended to be qualified under Section 401 of the Code is so qualified; each of the Company and its subsidiaries has fulfilled its obligations, if any, under Section 515 of ERISA; neither the Company nor any of its subsidiaries maintains or is required to contribute to a “welfare plan” (as defined in Section 3(1) of ERISA) which provides retiree or other post-employment welfare benefits or insurance coverage (other than “continuation coverage” (as defined in Section 602 of ERISA)); each pension plan and welfare plan established or maintained by the Company and/or one or more of its subsidiaries is in compliance in all material respects with the currently applicable provisions of ERISA; and neither the Company nor any of its subsidiaries has incurred or could reasonably be expected to incur any withdrawal liability under Section 4201 of ERISA, any liability under Section 4062, 4063, or 4064 of ERISA, or any other liability under Title IV of ERISA.

(kk) Neither the Company nor any of its subsidiaries nor, to the knowledge of the Company, any director, officer, agent, employee or affiliate of the Company or any of its subsidiaries is aware of or has taken any action, directly or indirectly, that would result in a violation by such persons of the FCPA, including, without limitation, making use of the mails or any means or instrumentality of interstate commerce corruptly in furtherance of an offer, payment, promise to pay or authorization of the payment of any money, or other property, gift, promise to give, or authorization of the giving of anything of value to any “foreign official” (as such term is defined in the FCPA) or any foreign political party or official thereof or any candidate for foreign political office, in contravention of the FCPA, and the Company, its subsidiaries and, to the knowledge of the Company, its affiliates have conducted their businesses in compliance with the FCPA and have instituted and maintain policies and procedures designed to ensure, and which are reasonably expected to continue to ensure, continued compliance therewith.

(ll) The operations of the Company and its subsidiaries are and have been conducted at all times in compliance with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all applicable jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or guidelines issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”) and no action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Company or any of its subsidiaries with respect to the Money Laundering Laws is pending or, to the best knowledge of the Company, threatened.

(mm) Neither the Company nor, to the knowledge of the Company, any director, officer, agent, employee or affiliate of the Company is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Treasury Department (“OFAC”); and the Company will not directly or indirectly use the proceeds

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of the offering, or lend, contribute or otherwise make available such proceeds, to any subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

(nn) Except as would not, individually or in the aggregate, have a Material Adverse Effect, the Company and its subsidiaries own, possess, license or have other rights to use, on reasonable terms, all patents, patent applications, trade and service marks, trade and service mark registrations, trade names, copyrights, licenses, inventions, trade secrets, technology, know-how and other intellectual property necessary for the conduct of their respective businesses as now conducted or as proposed in the Prospectus to be conducted. Neither the Company nor any of its subsidiaries has received any notice of any claim of infringement of or conflict with any such rights of others that if determined adversely to the Company or such subsidiary, would, individually or in the aggregate, have a Material Adverse Effect.

(oo) Neither the Company nor any subsidiary has taken, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Stock.

(pp) The Stock has been approved for listing, subject to official notice of issuance, on the New York Stock Exchange.

Any certificate signed by any officer of the Company and delivered to the Representatives or counsel for the Underwriters in connection with the offering of the Stock shall be deemed a representation and warranty by the Company, as to matters covered thereby, to each Underwriter.

2. Purchase and Sale .

(a) The Firm Stock . The Company agrees to issue and sell to the several Underwriters the Firm Stock upon the terms herein set forth. On the basis of the representations, warranties and agreements herein contained, and upon the terms bu