UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

DATE OF REPORT (Date of earliest event reported) August 1, 2013

TELLABS, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (630) 798-8800

N/A

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Item 2.02 Results of Operations and Financial Condition

On August 1, 2013, Tellabs, Inc. is reporting its results of operations for its fiscal second quarter ended June 28, 2013. A copy of the press release issued by Tellabs, Inc. concerning the foregoing results is furnished herewith as Exhibit 99.1.

The information contained in this Item 2.02 and related information in the accompanying Exhibit 99.1 shall not be incorporated by reference into any filing of Tellabs, Inc., whether made before or after the date hereof, regardless of any general incorporation language in such filing, unless expressly incorporated by specific reference to such filing. The information in this Item 2.02, including related information in Exhibit 99.1 hereto, shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or otherwise subject to the liabilities of that Section or Sections 11 and 12(a)(2) of the Securities Act of 1933, as amended.

In addition to reporting financial results in accordance with U.S. generally accepted accounting principles (GAAP), Tellabs, Inc. has provided non-GAAP financial measures in the press release attached as Exhibit 99.1 as additional information to evaluate our operating performance. These operating performance measures have not been prepared in accordance with GAAP and may be different from measures used by other companies. Whenever we use non-GAAP financial performance measures, we provide a reconciliation of non-GAAP financial performance measures to the most closely applicable GAAP financial performance measure. Management believes the non-GAAP financial performance measures are an important measure of operating performance and provides useful information to investors to better evaluate the operating performance of our business as it highlights trends in our business that may not otherwise be apparent when relying solely on GAAP measures and because it facilitates comparisons to historical results of operations. Management uses these non-GAAP financial performance measures as supplements to our GAAP results in order to provide a more complete understanding of the factors and trends affecting our business as well as for business planning and performance management. Management discloses this information publicly along with a reconciliation of the comparable GAAP amounts, to provide access to the detail and general nature of adjustments made to GAAP financial results. While some of these excluded items have been periodically reported in our statements of operations, including significant restructuring and other charges, their occurrence in future periods depends on future business and economic factors, among other evaluation criteria, and the occurrence of such events and factors may frequently be beyond the control of management.

Item 9.01 Financial Statements and Exhibits

(d) Exhibits.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

August 1, 2013

(Date)

EXHIBIT INDEX