UNDERWRITING AGREEMENT

Exhibit 1.1

10,000,000 Shares

SBA Communications Corporation

Class A Common Stock

UNDERWRITING AGREEMENT

September 29, 2005

Lehman Brothers Inc.

745 Seventh Avenue, Third Floor

New York, New York 10019

Ladies and Gentlemen:

SBA Communications Corporation, a Florida corporation (the “ Company ”) proposes to sell to Lehman Brothers Inc. (the “ Underwriter ”) 10,000,000 shares (the “ Stock ”) of the vCompany’s Class A Common Stock, par value $0.01 per share (the “ Common Stock ”). This is to confirm the agreement concerning the purchase of the Stock from the Company by the Underwriter.

1. Representations, Warranties and Agreements of the Company. The Company represents, warrants and agrees as follows:

(a) A registration statement on Form S-3 with respect to the Stock has (i) been prepared by the Company in conformity with the requirements of the Securities Act of 1933, as amended (the “ Securities Act ”), and the rules and regulations (the “ Rules and Regulations ”) of the Securities and Exchange Commission (the “ Commission ”) thereunder, (ii) been filed with the Commission under the Securities Act and (iii) become effective under the Securities Act. Copies of such registration statement and each of the amendments thereto have been delivered by the Company to you. As used in this Agreement, “ Effective Time ” means the date and the time as of which such registration statement, or the most recent post-effective amendment thereto, if any, was declared effective by the Commission; “ Effective Date ” means the date of the Effective Time; “ Preliminary Prospectus Supplement ” means each preliminary prospectus supplement amending or supplementing the Base Prospectus (as defined below) after the registration statement became effective under the Securities Act but containing a “Subject to Completion” legend comparable to that contained in paragraph 10 of Item 501 under Regulation S-K of the Rules and Regulations; “Preliminary Prospectus” means the Base Prospectus and each Preliminary Prospectus Supplement”; “Prospectus Supplement” means the final prospectus supplement amending or supplementing the Base Prospectus in the form filed after the Effective Time pursuant to Rule 424 of the Rules and Regulations and first used in connection with sales of the Stock; “ Prospectus ” means the Prospectus Supplement and the accompanying base prospectus included in the Registration Statement (the “Base Prospectus”) in the form first used in connection with sales of the Stock; and “ Registration Statement ” means the effective

registration statement on Form S-3 (Registration Statement No. 333-41308), as amended at the Effective Time, including all documents incorporated by reference therein at such time and all information contained in the Prospectus. Reference made herein to any Preliminary Prospectus Supplement, Preliminary Prospectus, Base Prospectus or to the Prospectus shall be deemed to refer to and include any documents incorporated by reference therein pursuant to Item 12 of Form S-3 under the Securities Act, as of the date of such Preliminary Prospectus or the Prospectus, as the case may be, and any reference to any amendment or supplement to any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include any document filed under the Securities Exchange Act of 1934, as amended (the “ Exchange Act ”), after the date of such Preliminary Prospectus or the Prospectus, as the case may be, and incorporated by reference in such Preliminary Prospectus or the Prospectus, as the case may be; and any reference to any amendment to the Registration Statement shall be deemed to include any annual report of the Company filed with the Commission pursuant to Section 13(a) or 15(d) of the Exchange Act after the Effective Time that is incorporated by reference in the Registration Statement. If the Company has filed an abbreviated registration statement to register additional shares of Common Stock pursuant to Rule 462(b) under the Securities Act (the “ Rule 462 Registration Statement ”), then any reference herein to the term “ Registration Statement ” shall be deemed to include such Rule 462 Registration Statement. The Commission has not issued any order preventing or suspending the use of any Preliminary Prospectus or Prospectus, or suspending the effectiveness of the Registration Statement, and no proceedings for such purpose have been instituted or, to the knowledge of the Company, threatened by the Commission.

(b) As of the applicable effective date of the Registration Statement and any amendment thereto, the Registration Statement and any amendment thereto complied in all material respects with the Securities Act and the Rules and Regulations, and did not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary in order to make the statements therein not misleading; and as of the applicable filing date of the Prospectus and any amendment or supplement thereto and as of the Delivery Date (as defined below) the Prospectus did not and will not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided that no representation or warranty is made as to information contained in or omitted from the Registration Statement or the Prospectus in reliance upon and in conformity with written information furnished to the Company by or on behalf of the Underwriter specifically for inclusion therein.

(c) The documents incorporated by reference in the Prospectus, 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 Exchange Act and the rules and regulations of the Commission thereunder, and none of such documents contained any untrue statement of a material fact or omitted to state any material fact required to be stated therein or necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; and any further documents so filed and incorporated by reference in the Prospectus, when such documents are filed with Commission will conform in all material respects to the requirements of the Exchange Act and the rules and regulations of the Commission thereunder 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, in the light of the circumstances under which they were made, not misleading.

(d) The Company is duly incorporated and validly existing and in good standing under the laws of Florida with all requisite corporate power and authority to own, lease and operate its properties and to conduct its business as described in the Prospectus, and is duly registered and qualified to conduct its business and is in good standing in each jurisdiction or place where the nature of its properties or the conduct of its business requires such registration or qualification, and none of the subsidiaries (as defined in Rule 405 under the Securities Act) (“ Subsidiaries ”) of the Company other than SBA Telecommunications, Inc. (“ Telecommunications ”), SBA Senior Finance Inc., SBA Properties, Inc., SBA Network Services, Inc. and SBA Towers, Inc. (collectively, the “ Significant Subsidiaries ”) is a “significant subsidiary” as such term is defined in Rule 405 under the Securities Act.

(e) The Company has an authorized capitalization as set forth in the Prospectus, and all of the issued shares of capital stock of the Company have been duly and validly authorized and issued, are fully paid and non-assessable and conform in all material respects to the description thereof contained in the Prospectus.

(f) Each of the Subsidiaries of the Company is duly organized and validly existing and in good standing under the laws of the jurisdiction of its organization, with all requisite power and authority to own, lease and operate its properties and is duly registered and qualified to conduct its business and is in good standing in each jurisdiction or place where the nature of its properties or the conduct of its business requires such registration or qualification, except where the failure to be duly registered or qualified would not have caused a material adverse effect on the general affairs, management, consolidated financial position, stockholders’ equity, results of operations, business or prospects of the Company and the Subsidiaries taken as a whole (a “ Material Adverse Effect ”). The Company and the Subsidiaries, as a whole, conduct their business as described in the Prospectus. All of the issued shares of capital stock of each Subsidiary of the Company have been duly authorized and validly issued and are fully paid and non-assessable, are owned directly or indirectly by the Company, and (except as set forth in the Prospectus with respect to shares subject to liens under or pursuant to the Amended and Restated Credit Agreement, dated as of January 30, 2004, as further amended on November 12, 2004, by and among SBA Senior Finance, Inc., the several lenders from time to time parties thereto and Lehman Brothers Inc. and Deutsche Bank Securities Inc., as joint lead advisors, arrangers and bookrunners, as amended, supplemented or otherwise modified from time to time, the “ Senior Credit Agreement ”) free and clear of all liens, encumbrances, equities, claims or adverse interests.

(g) The shares of the Stock to be issued and sold by the Company to the Underwriter hereunder have been duly and validly authorized and, when issued and delivered against payment therefor as provided herein will be duly and validly issued, fully paid and non-assessable; and the Stock will conform to the description thereof contained in the Prospectus.

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

(i) The Company has all requisite corporate power and authority to execute, deliver and perform its obligations under this Agreement.

(j) The execution, delivery and performance of this Agreement by the Company and the consummation of the transactions contemplated hereby will not conflict with, or result in a breach or violation of any of the terms or provisions of, or (including with the giving of notice or the lapse of time or both) constitute a default under (i) any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which the Company or any of its Subsidiaries is a party or by which the Company or any of its Subsidiaries is bound or to which any of the properties or assets of the Company or any of its Subsidiaries is subject, (ii) the provisions of the charter, by-laws or other organizational documents of the Company or any of its Subsidiaries or (iii) any statute or any order, rule or regulation of any court or governmental agency or body having jurisdiction over the Company or any of its Subsidiaries or any of their properties or assets, except in the cases of clause (i) or (iii), such breaches, violations or defaults that in the aggregate would not have a Material Adverse Effect, and no consent, approval, authorization or order of, or filing or registration with, any court or governmental agency or body is required for the execution, delivery and performance of this Agreement by the Company and the consummation of the transactions contemplated hereby except for the registration of the Stock under the Securities Act, such consents, approvals, authorizations, registrations or qualifications as may be required under the Exchange Act and applicable state or foreign securities laws in connection with the purchase and distribution of the Stock by the Underwriter and the listing of the Stock on the Nasdaq National Market System as may be required.

(k) Except (i) as described in the Prospectus and (ii) for the registration rights granted pursuant to the Registration Rights Agreement dated as of December 14, 2004, among the Company, Lehman Brothers Inc., Deutsche Bank Securities Inc., Friedman, Billings, Ramsey & Co., Inc., Raymond James & Associates, Inc., RBC Capital Markets Corporation and TD Securities (USA) LLC, there are no contracts, agreements or understandings between the Company or any of its Subsidiaries, on the one hand, and any person, on the other hand, granting such person the right to require the Company or any of its Subsidiaries to file a registration statement under the Securities Act with respect to any securities of the Company and its Subsidiaries owned or to be owned by such person or to require the Company or any of its Subsidiaries to include such securities in the securities registered pursuant to the Registration Statement or in any securities being registered pursuant to any other registration statement filed by the Company or any of its Subsidiaries under the Securities Act.

(l) The Company has not sold or issued any shares of Common Stock during the six-month period preceding the date of the Prospectus, including any sales pursuant to Rule 144A under, or Regulations D or S of, the Securities Act other than shares issued pursuant to employee benefit plans, qualified stock options plans or other employee compensation plans or pursuant to outstanding options, rights or warrants, shares issued pursuant to the registration statements on Form S-4 (File Nos. 333-71460 and 333-46730) and shares of common stock issued pursuant to Section 3(a)(9) of the Securities Act in connection with debt-for-equity transactions.

(m) Neither the Company nor any of its Subsidiaries has sustained, since the date of the latest audited financial statements included or incorporated by reference in the Prospectus, any material loss or interference with its business from fire, explosion, flood or other calamity, whether or not covered by insurance, or from any labor dispute or court or governmental action, order or decree, otherwise than as set forth or contemplated in the Prospectus; and, since such date, there has not been any change in the capital stock or long-term debt of the Company on a consolidated basis or any material adverse change, or any development involving a prospective material adverse change, in or affecting the general affairs, management, consolidated financial position, stockholders’ equity, results of operations, business or prospects of the Company and its Subsidiaries taken as a whole, in each case otherwise than as set forth or contemplated in the Prospectus.

(n) The consolidated historical financial statements, together with the related notes thereto, included or incorporated by reference in the Registration Statement and the Prospectus comply as to form in all material respects with the requirements of Regulation S-X under the Securities Act applicable to registration statements on Form S-3 under the Securities Act. Such historical financial statements fairly present the financial position of the Company at the respective dates indicated and the results of operations and cash flows for the respective periods indicated, in each case in accordance with generally accepted accounting principles (“ GAAP ”) consistently applied throughout such periods. The other financial information and data filed as part of the Registration Statement or included or incorporated by reference in the Prospectus are, in all material respects, accurately presented and prepared on a basis consistent with such financial statements and the books and records of the Company.

(o) Ernst & Young LLP, who have certified certain financial statements of the Company, whose report appears in the Form 10-K incorporated by reference into the Prospectus and who have delivered the initial letter referred to in Section 7(g) hereof, are independent public accountants as required by the Securities Act and the Rules and Regulations and were independent accountants under the guidelines of the AICPA as required by the Securities Act and the Rules and Regulations during the periods covered by the financial statements on which they reported incorporated by reference into the Prospectus.

(p) The Company and each of its Subsidiaries have good and marketable title in fee simple to or a leasehold interest in all real property and good and valid title to all personal property owned by them, in each case free and clear of all liens, encumbrances, defects, equities or claims except for liens contemplated by the Senior Credit Agreement or as are otherwise described in the Prospectus or such as do not materially affect the value of such property and do not materially interfere with the use made and proposed to be made of such property by the Company and its Subsidiaries; all assets held under lease by the Company and its Subsidiaries are held by them under valid, subsisting and enforceable leases, with such exceptions as are not material and do not materially interfere with the use made and proposed to be made of such assets by the Company and its Subsidiaries taken as a whole; and the present and contemplated use of the assets owned or leased by the Company or any of its Subsidiaries for the operation of towers is in compliance in all material respects with all applicable zoning ordinances and regulations and other laws and regulations where failure so to comply would result, or create reasonable risk of resulting, in a Material Adverse Effect.

(q) The Company and each of its Subsidiaries carry, or are covered by, insurance in such amounts and covering such risks as is adequate for the conduct of their respective businesses and the value of their respective properties and as is customary for companies engaged in similar businesses in similar industries.

(r) The Company and each of its Subsidiaries own or possess adequate rights to use all patents, patent applications, trademarks, service marks, trade names, trademark registrations, service mark registrations, copyrights, inventions, know-how (including trade secrets and other unpatented and/or unpatentable proprietary or confidential information, systems or procedures) and licenses necessary for the conduct of their respective businesses and have no reason to believe that the conduct of their respective businesses will conflict with, and have not received any notice of any claim of conflict with, any such rights of others, in each case except as could not reasonably be expected to have a Material Adverse Effect.

(s) There are no legal or governmental proceedings pending or, to the knowledge of the Company or its Subsidiaries, threatened against the Company or any of its Subsidiaries or to which any of their respective properties is subject, that are not disclosed in the Prospectus and which are reasonably likely to have a Material Adverse Effect or to materially affect the issuance of the Stock.

(t) The conditions for use of Form S-3, as set forth in the General Instructions thereto, have been satisfied.

(u) There are no contracts or other documents which are required to be described in the Prospectus or filed as exhibits to the Registration Statement by the Securities Act or by the Rules and Regulations which have not been described in the Prospectus or filed as exhibits to the Registration Statement or incorporated therein by reference as permitted by the Rules and Regulations.

(v) No material relationship, direct or indirect, exists between or among the Company or Telecommunications on the one hand, and the directors, officers, stockholders, affiliates, customers or suppliers of the Company or Telecommunications on the other hand, which is required to be described in the Prospectus which is not so described.

(w) Neither the Company nor any of its Subsidiaries is involved in any strike or labor dispute with any group of employees, and, to the knowledge of the Company or any of its Subsidiaries, no such action or dispute is threatened, which might be expected to have a Material Adverse Effect.

(x) The Company is in compliance in all material respects with all presently applicable provisions of the Employee Retirement Income Security Act of 1974, as amended, including the regulations and published interpretations thereunder (“ ERISA ”); no “reportable event” (as defined in ERISA) has occurred with respect to any “pension plan” (as defined in ERISA) for which the Company would have any liability; the Company has not incurred nor expects to incur liability under (i) Title IV of ERISA with respect to termination of, or

withdrawal from, any “pension plan” or (ii) Sections 412 or 4971 of the Internal Revenue Code of 1986, as amended, including the regulations and published interpretations thereunder (the “ Code ”); each “pension plan” for which the Company would have any liability that is intended to be qualified under Section 401(a) of the Code is so qualified in all material respects and nothing has occurred, whether by action or by failure to act, which would cause the loss of such qualification.

(y) Each of the Company and its Subsidiaries has filed all federal, state and local income and franchise tax returns required to be filed through the date hereof and has paid all taxes due thereon except where such failure would not have a Material Adverse Effect, and no tax deficiency has been determined adversely to the Company or any of its Subsidiaries nor does the Company or any of its Subsidiaries have any knowledge of any tax deficiency which, if determined adversely to the Company, would have a Material Adverse Effect.

(z) Since the date of the latest audited consolidated financial statements of the Company incorporated by reference in the Prospectus, neither the Company nor any of its Subsidiaries has incurred any liability or obligation, direct or contingent, or entered into any transaction, in each case not in the ordinary course of business, and that is material to the Company and its Subsidiaries, taken as a whole, and there has been no Material Adverse Effect, nor to the Company’s knowledge, after due inquiry, any development or event involving a prospective Material Adverse Effect and, except as disclosed in or contemplated by the Prospectus, since the date of the latest audited consolidated financial statements of the Company incorporated by reference in the Prospectus, there has been no (i) dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock, (ii) issuance of securities, other than the securities issued pursuant to the Company’s 1999 Equity Participation Plan, the Company’s 2001 Equity Participation Plan, the Company’s 1999 Employee Stock Purchase Plan, shares of common stock of the Company issued pursuant to the registration statements on Form S-4 (File Nos. 333-71460 and 333-46730) and shares of common stock of the Company issued pursuant to Section 3(a)(9) of the Securities Act in connection with debt-for-equity transactions, or (iii) material increase in short-term or long-term debt of the Company on a consolidated basis.

(aa) The Company and its Subsidiaries have a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorization; (ii) transactions are recorded as necessary to permit preparation of its consolidated financial statements in conformity with GAAP and to maintain accountability for assets; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the reported accountability for assets is compared with existing assets at reasonable intervals.

(bb) The Company has established and maintains disclosure controls and procedures (as such term is defined in Rule 13a-14 under the Exchange Act), which (i) are designed to ensure that material information relating to the Company, including its consolidated subsidiaries, is made known to the Company’s principal executive officer and its principal financial officer or persons performing similar functions by others within the Company, particularly during the periods in which the periodic reports required under the Exchange Act are being prepared; (ii)

have been evaluated for effectiveness as of December 31, 2004; and (iii) are effective in all material respects to perform the functions for which they were established except as described in the Form 10-K incorporated by reference in the Prospectus.

(cc) Based on the evaluation of its disclosure controls and procedures, the Company is not aware of (i) any significant deficiency in the design or operation of internal controls which could adversely affect the Company’s ability to record, process, summarize and report financial data or any material weaknesses in internal controls, except as described in the Form 10-K incorporated by reference in the Prospectus or (ii) any fraud, whether or not material, that involves management or other employees who have a significant role in the Company’s internal controls.

(dd) Since the date of the most recent evaluation of such disclosure controls and procedures, there have been no significant changes in internal controls or in other factors that could significantly affect internal controls, including any corrective actions with regard to significant deficiencies and material weaknesses, except as described in the Form 10-K incorporated by reference in the Prospectus.

(ee) There is and has been no failure on the part of the Company and any of its directors or officers, in their capacities as such, to comply with any applicable provision of the U.S. Sarbanes Oxley Act of 2002 and the rules and regulations promulgated in connection therewith, other than such failures relating to the Company’s assessment and evaluation of its application of lease accounting and leasehold depreciation practices and the resulting conclusion: (i) that the Company’s disclosure controls and procedures were not effective as of December 31, 2004; (ii) that a material weakness in the Company’s internal control over financial reporting existed as of December 31, 2004; and (iii) that the Company’s internal control over financial reporting was not effective as of December 31, 2004.

(ff) Neither the Company nor any of the Significant Subsidiaries (i) is in violation of its charter or by-laws, (ii) is in default, and no event has occurred which, with notice or lapse of time or both, would constitute such a default, in the due performance or observance of any term, covenant or condition contained in any indenture, mortgage, deed of trust, loan agreement or other agreement or instrument to which it is a party or by which it is bound or to which any of its properties or assets is subject or (iii) is in violation of any statute or any order, rule or regulation of any court or governmental agency or body having jurisdiction over the Company or any of its Subsidiaries or any of their properties or assets, other than, a default or violation described in clauses (ii) and (iii) which is not reasonably likely to have a Material Adverse Effect.

(gg) Neither the Company nor any of its Subsidiaries, nor any director, officer, agent, employee or other person associated with or acting on behalf of the Company or any of its Subsidiaries, has used any corporate funds for any unlawful contribution, gift, entertainment or other unlawful expense relating to political activity; made any direct or indirect unlawful payment to any foreign or domestic government official or employee from corporate funds; violated or is in violation of any provision of the Foreign Corrupt Practices Act of 1977; or made any bribe, rebate, payoff, influence payment, kickback or other unlawful payment.

(hh) There has been no storage, disposal, generation, manufacture, refinement, transportation, handling or treatment of toxic wastes, hazardous wastes or hazardous substances by the Company or any of its Subsidiaries (or, to the knowledge of the Company, any of their predecessors in interest) at, upon or from any of the property now or previously owned or leased by the Company or any of its Subsidiaries in violation of any applicable law, ordinance, rule, regulation, order, judgment, decree or permit or which would require remedial action under any applicable law, ordinance, rule, regulation, order, judgment, decree or permit, except for any violation or remedial action which would not have, or could not be reasonably likely to have, singularly or in the aggregate with all such violations and remedial actions, a Material Adverse Effect; there has been no material spill, discharge, leak, emission, injection, escape, dumping or release of any kind onto such property or into the environment surrounding such property of any toxic wastes, medical wastes, solid wastes, hazardous wastes or hazardous substances due to or caused by the Company or any of its Subsidiaries or with respect to which the Company or any of its Subsidiaries has knowledge, except for any such spill, discharge, leak, emission, injection, escape, dumping or release which would not have or would not be reasonably likely to have, singularly or in the aggregate with all such spills, discharges, leaks, emissions, injections, escapes, dumpings and releases, a Material Adverse Effect; and the terms “hazardous wastes,” “toxic wastes,” “hazardous substances” and “medical wastes” shall have the meanings specified in any applicable local, state, Federal and foreign laws or regulations with respect to environmental protection.

(ii) Neither the Company nor any Subsidiary is currently or will be, upon the sale of the Stock in accordance herewith and the application of the net proceeds therefrom as described in the Prospectus under the caption “Use of Proceeds,” an “investment company” within the meaning of and subject to regulation under the Investment Company Act of 1940, as amended (the “ 1940 Act ”).

(jj) The Company and its Subsidiaries (i) have duly and timely filed all material reports, registrations and other material filings, if any, which are required to be filed by it or any of its Subsidiaries under the Communications Act of 1934, any similar or successor federal statute, and the rules of the Federal Communications Commission (“ FCC ”) thereunder or any other applicable law, rule or regulation of any governmental authority, including the FCC and the Federal Aviation Authority (“ FAA ”), other than such filings for which the failure to file would not result, or would not be reasonably likely to result, in a Material