Capital Securities Guarantee Agreement

Exhibit 4.3

Capital Securities Guarantee Agreement

First Midwest Bancorp, Inc.

Dated as of August 21, 2009

Table of Contents

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Capital Securities Guarantee Agreement

This Guarantee Agreement (the “ Capital Securities Guarantee ”), dated as of August 21, 2009, is executed and delivered by First Midwest Bancorp, Inc., a Delaware corporation (the “ Guarantor ”), and Wilmington Trust Company, a Delaware banking corporation, as trustee (the “ Capital Securities Guarantee Trustee ”), for the benefit of the Holders (as defined herein) from time to time of the Capital Securities (as defined herein) of First Midwest Capital Trust I, a Delaware statutory trust, formerly known as New First Midwest Capital Trust I (the “ Issuer ”).

Witnesseth:

Whereas, pursuant to an Amended and Restated Declaration of Trust (the “ Declaration ”), dated as of August 21, 2009, among the trustees of the Issuer, the Guarantor, as sponsor, and the holders from time to time of undivided beneficial interests in the assets of the Issuer and an Agreement of Merger, dated as of August 21, 2009 (the “Agreement of Merger”), between the Issuer and the Predecessor Issuer (as defined herein) the Issuer has outstanding on the date hereof 125,000 capital securities, having an aggregate liquidation amount of $125,000,000, such capital securities being designated the 6.95% Capital Securities (collectively the “ Capital Securities ”); and

Whereas, the Guarantor desires irrevocably and unconditionally to agree, to the extent set forth in this Capital Securities Guarantee, to pay to the Holders the Guarantee Payments (as defined below) and to make certain other payments on the terms and conditions set forth herein;

Now, Therefore, in consideration of the premises, the Guarantor executes and delivers this Capital Securities Guarantee for the benefit of the Holders.

Article I

Definitions and Interpretation

Section 1.1. Definitions and Interpretation . In this Capital Securities Guarantee, unless the context otherwise requires:

(a) capitalized terms used in this Capital Securities Guarantee but not defined in the preamble above have the respective meanings assigned to them in this Section 1.1;

(b) terms defined in the Declaration as of the date of execution of this Capital Securities Guarantee have the same meaning when used in this Capital Securities Guarantee unless otherwise defined in this Capital Securities Guarantee;

(c) a term defined anywhere in this Capital Securities Guarantee has the same meaning throughout;

(d) all references to “the Capital Securities Guarantee” or “this Capital Securities Guarantee” are to this Capital Securities Guarantee as modified, supplemented or amended from time to time;

(e) all references in this Capital Securities Guarantee to Articles and Sections are to Articles and Sections of this Capital Securities Guarantee, unless otherwise specified;

(f) a term defined in the Trust Indenture Act has the same meaning when used in this Capital Securities Guarantee, unless otherwise defined in this Capital Securities Guarantee or unless the context otherwise requires; and

(g) a reference to the singular includes the plural and vice versa.

“ Affiliate ” has the same meaning as given to that term in Rule 405 under the Securities Act of 1933, as amended, or any successor rule thereunder.

“ Business Day ” means any day other than a Saturday or a Sunday, or a day on which banking institutions in The City of New York are authorized or required by law, executive order or regulation to close or a day on which the Corporate Trust Office of the Capital Securities Guarantee Trustee is closed for business.

“ Capital Securities Guarantee Trustee ” means Wilmington Trust Company, a Delaware banking corporation, until a Successor Capital Securities Guarantee Trustee has been appointed and has accepted such appointment pursuant to the terms of this Capital Securities Guarantee and thereafter means each such Successor Capital Securities Guarantee Trustee.

“ Common Securities ” means the securities representing common undivided beneficial interests in the assets of the Issuer.

“ Corporate Trust Office ” means the office of the Capital Securities Guarantee Trustee at which the corporate trust business of the Capital Securities Guarantee Trustee shall, at any particular time, be principally administered, which office at the date of execution of this Agreement is located at Rodney Square North, 1100 North Market Street, Wilmington, Delaware 19890-0001, Attention: Corporate Trust Administration.

“ Covered Person ” means any Holder or beneficial owner of Capital Securities.

“ Debentures ” means the series of subordinated debt securities of the Guarantor designated the 6.95% Series B Junior Subordinated Deferrable Interest Debentures due December 1, 2033 held by the Property Trustee (as defined in the Declaration) of the Issuer.

“ Event of Default ” means a default by the Guarantor on any of its payment or other obligations under this Capital Securities Guarantee.

“ Guarantee Payments ” means the following payments or distributions, without duplication, with respect to the Capital Securities, to the extent not paid by or on behalf of the Issuer: (i) any accumulated and unpaid Distributions (as defined in the Declaration) that are required to be paid on such Capital Securities, to the extent the Issuer has funds on hand legally

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available therefor at such time, (ii) the redemption price, including all accumulated and unpaid Distributions to the date of redemption (the “ Redemption Price ”), to the extent the Issuer has funds on hand legally available therefor at such time, with respect to any Capital Securities called for redemption by the Issuer, and (iii) upon a voluntary or involuntary dissolution, winding up or liquidation of the Issuer (other than in connection with the distribution of Debentures to the Holders in exchange for Capital Securities as provided in the Declaration), the lesser of (a) the aggregate of the liquidation amount and all accumulated and unpaid Distributions on the Capital Securities to the date of payment, to the extent the Issuer has funds on hand legally available therefor, and (b) the amount of assets of the Issuer remaining available for distribution to Holders upon liquidation of the Issuer after satisfaction of liabilities to creditors of the Issuer as required by applicable law.

“ Holder ” shall mean any holder, as registered on the books and records of the Issuer, of any Capital Securities; provided , however , that, in determining whether the holders of the requisite percentage of Capital Securities have given any request, notice, consent or waiver hereunder, “Holder” shall not include the Guarantor or any Affiliate of the Guarantor.

“ Indemnified Person ” means the Capital Securities Guarantee Trustee, any Affiliate of the Capital Securities Guarantee Trustee, or any officers, directors, shareholders, members, partners, employees, representatives, nominees, custodians or agents of the Capital Securities Guarantee Trustee.

“ Indenture ” means the Indenture dated as of November 18, 2003, as amended and supplemented, among the Guarantor (the “ Debenture Issuer ”) and Wilmington Trust Company, as trustee, pursuant to which the Debentures are issued and outstanding.

“ Majority in liquidation amount of the Capital Securities ” means, except as provided by the Trust Indenture Act, a vote by Holder(s) of more than 50% of the aggregate liquidation amount (including the stated amount that would be paid on redemption, liquidation or otherwise, plus accumulated and unpaid Distributions to the date upon which the voting percentages are determined) of all outstanding Capital Securities.

“ Officers’ Certificate ” means, with respect to the Guarantor, a certificate signed by the Chairman, the Chief Executive Officer, the President, a Vice President, the Chief Financial Officer, the Secretary or an Assistant Secretary, of the Guarantor. Any Officers’ Certificate delivered with respect to compliance with a condition or covenant provided for in this Capital Securities Guarantee (other than pursuant to Section 314(a)(4) of the Trust Indenture Act) shall include:

(a) a statement that each officer signing the Officers’ Certificate has read the covenant or condition and the definitions relating thereto;

(b) a brief statement of the nature and scope of the examination or investigation undertaken by the officer rendering the Officers’ Certificate;

(c) a statement that each such officer has made such examination or investigation as, in such officer’s opinion, is necessary to enable such officer to express an informed opinion as to whether or not such covenant or condition has been complied with; and

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(d) a statement as to whether, in the opinion of each such officer, such condition or covenant has been complied with.

“ Other Debentures ” means all junior subordinated debentures issued by the Guarantor from time to time and sold to trusts, in each case similar to the Issuer, to be established by the Guarantor (if any).

“ Other Guarantees ” means all guarantees to be issued by the Guarantor with respect to capital securities (if any) similar to the Capital Securities issued by other trusts, in each case similar to the Issuer established by the Guarantor (if any).

“ Person ” means a legal person, including any individual, corporation, estate, partnership, joint venture, association, joint stock company, limited liability company, trust, unincorporated association, or government or any agency or political subdivision thereof, or any other entity of whatever nature.

“ Predecessor Issuer ” means First Midwest Capital Trust I, a Delaware statutory trust created pursuant to the Delaware Statutory Trust Act by the entering into of that certain Declaration of Trust, dated as of November 6, 2003, as amended and restated in its entirety by the Amended and Restated Declaration of Trust, dated November 18, 2003, and by the execution and filing by Wilmington Trust Company , as Trustee, with the Secretary of State of the State of Delaware of the Certificate of Trust, filed on November 6, 2003.

“ Responsible Officer ” means, when used with respect to the Capital Securities Guarantee Trustee, any officer of the Capital Securities Guarantee Trustee with direct responsibility for the administration of this Capital Securities Guarantee and also means, with respect to a particular corporate trust matter, any other officer to whom such matter is referred because of that Officer’s knowledge of and familiarity with the particular subject.

“ Successor Capital Securities Guarantee Trustee ” means a successor Capital Securities Guarantee Trustee possessing the qualifications to act as Capital Securities Guarantee Trustee under Section 4.1.

“ Trust Indenture Act ” means the Trust Indenture Act of 1939, as amended.

“ Trust Securities ” means the Common Securities and the Capital Securities, collectively.

Article II

Trust Indenture Act

Section 2.1. Trust Indenture Act; Application. (a) This Capital Securities Guarantee is subject to the provisions of the Trust Indenture Act that are required to be part of this Capital Securities Guarantee and shall, to the extent applicable, be governed by such provisions;

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(b) the Capital Securities Guarantee Trustee shall be the only trustee for purposes of the Trust Indenture Act; and

(c) if and to the extent that any provision of this Capital Securities Guarantee limits, qualifies or conflicts with the duties imposed by Section 310 to 317, inclusive, of the Trust Indenture Act, such imposed duties shall control.

Section 2.2. Lists of Holders of Securities. (a) The Guarantor shall provide the Capital Securities Guarantee Trustee (unless the Capital Securities Guarantee Trustee is otherwise the registrar of the Capital Securities) with a list, in such form as the Capital Securities Guarantee Trustee may reasonably require, of the names and addresses of the Holders (“ List of Holders ”) as of such date, (i) within one Business Day after May 1 and November 1 of each year, and (ii) at any other time within 30 days of receipt by the Guarantor of a written request for a List of Holders as of a date no more than 14 days before such List of Holders is given to the Capital Securities Guarantee Trustee; provided , that the Guarantor shall not be obligated to provide such List of Holders at any time the List of Holders does not differ from the most recent List of Holders given to the Capital Securities Guarantee Trustee by the Guarantor. The Capital Securities Guarantee Trustee may destroy any List of Holders previously given to it on receipt of a new List of Holders.

(b) The Capital Securities Guarantee Trustee shall comply with its obligations under Sections 311(a), 311(b) and Section 312(b) of the Trust Indenture Act.

Section 2.3. Reports by the Capital Securities Guarantee Trustee. Within 60 days after May 15 of each year, commencing May 15, 2010, the Capital Securities Guarantee Trustee shall provide to the Holders such reports as are required by Section 313(a) of the Trust Indenture Act, if any, in the form and in the manner provided by Section 313 of the Trust Indenture Act. The Capital Securities Guarantee Trustee shall also comply with the other requirements of Section 313 of the Trust Indenture Act.

Section 2.4. Periodic Reports to Capital Securities Guarantee Trustee. The Guarantor shall provide to the Capital Securities Guarantee Trustee such documents, reports and information as required by Section 314 of the Trust Indenture Act, if any, and the compliance certificate required by Section 314 of the Trust Indenture Act in the form, in the manner and at the times required by Section 314(a)(4) of the Trust Indenture Act; provided that such compliance certificate shall be delivered on or before 120 days after the end of each fiscal year of the Guarantor. Delivery of such documents, reports and information to the Capital Securities Guarantee Trustee is for informational purposes only and the Capital Securities Guarantee Trustee’s receipt of such shall not constitute constructive notice of any information contained therein or determinable from information contained therein, including the Guarantor’s compliance with any of its covenants hereunder (as to which the Capital Securities Guarantee Trustee is entitled to rely exclusively on Officers’ Certificates).

Section 2.5. Evidence of Compliance with Conditions Precedent. The Guarantor shall provide to the Capital Securities Guarantee Trustee such evidence of compliance with any conditions precedent, if any, provided for in this Capital Securities Guarantee that relate to any of the matters set forth in Section 314(c) of the Trust Indenture Act. Any certificate or opinion required to be given by an officer pursuant to Section 314(c)(1) may be given in the form of an Officers’ Certificate.

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Section 2.6. Events of Default; Waiver. The Holders of a Majority in liquidation amount of Capital Securities may, by vote, on behalf of all the Holders, waive any past Event of Default and its consequences. Upon such waiver, any such Event of Default shall cease to exist, and any Event of Default arising therefrom shall be deemed to have been cured, for every purpose of this Capital Securities Guarantee, but no such waiver shall extend to any subsequent or other Event of Default or impair any right consequent thereon.

Section 2.7. Event of Default; Notice. (a) The Capital Securities Guarantee Truste