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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:16.0pt;font-weight:bold;">UNITED   STATES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:16.0pt;font-weight:bold;">SECURITIES   AND EXCHANGE COMMISSION</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">WASHINGTON,   D.C.&nbsp; 20549</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:16.0pt;font-weight:bold;">FORM 8-K</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">CURRENT&nbsp; REPORT</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Pursuant to Section 13 or 15(d) of   the Securities Exchange Act of 1934</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date   of Report (Date of earliest event reported):</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">June 18, 2013</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:16.0pt;font-weight:bold;">TETRA   TECH, INC.</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Exact name of registrant   as specified in its charter)</font></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Delaware</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">0-19655</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">95-4148514</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(State or other   jurisdiction <br>   of incorporation)</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(IRS Employer</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Identification&nbsp;Number)</font></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">3475 East Foothill Boulevard,   Pasadena, California&nbsp; 91107</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Address of principal   executive office, including zip code)</font></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">(626) 351-4664</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Registrant's telephone   number, including area code)</font></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Not Applicable</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Former name or former address, if changed since   last report)</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;text-align:justify;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">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:</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Wingdings" style="font-size:12.0pt;">o</font><font size="1" style="font-size:8.5pt;">&nbsp;&nbsp; </font><font size="2" style="font-size:10.0pt;">Written   communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Wingdings" style="font-size:12.0pt;">o</font><font size="1" style="font-size:8.5pt;">&nbsp;&nbsp; </font><font size="2" style="font-size:10.0pt;">Soliciting   material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="3" face="Wingdings" style="font-size:12.0pt;">o</font><font size="1" style="font-size:8.5pt;">&nbsp;&nbsp; </font><font size="2" style="font-size:10.0pt;">Pre-commencement   communications pursuant to Rule 14d-2(b) under the Exchange Act   (17&nbsp;CFR&nbsp;240.14d-2(b))</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="3" face="Wingdings" style="font-size:12.0pt;">o</font><font size="1" style="font-size:8.5pt;">&nbsp;&nbsp; </font><font size="2" style="font-size:10.0pt;">Pre-commencement   communications pursuant to Rule 13e-4(c) under the Exchange Act   (17&nbsp;CFR&nbsp;240.13e-4(c))</font></p>    </td>   </tr>  </table>
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<h6 style="font-weight:bold;margin:0in 0in .0001pt;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;">Item 7.01.&#160;&#160;&#160;&#160;&#160;&#160; Regulation FD Disclosure.</font></b></h6>
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<p style="margin:0in 0in .0001pt;text-align:justify;text-indent:.5in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">On June 17, 2013, the Board of Directors of Tetra Tech, Inc. (the &#147;Registrant&#148;) authorized the repurchase of up to $100 million of the Registrant&#146;s common stock (the &#147;Stock Repurchase Program&#148;).&#160; A copy of the press release issued by the Registrant concerning the Stock Repurchase Program and the event described in Item 8.01 below is furnished herewith as Exhibit 99.1 and is incorporated herein by reference in its entirety.</font></p>
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<h6 style="font-weight:bold;margin:0in 0in .0001pt;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;">Item 8.01.&#160;&#160;&#160;&#160;&#160;&#160; Other Events.</font></b></h6>
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<p style="font-size:12.0pt;margin:0in 0in .0001pt;text-align:justify;text-indent:.5in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">On June 18, 2013, the Registrant issued a press release that updated its third quarter 2013 outlook </font>due to increased restructuring costs resulting from the previously announced weakness in Eastern Canada and mining, and new findings on certain project claims.&#160; A copy of the press release issued by the Registrant concerning the foregoing and the event described in Item 7.01 above is furnished herewith as Exhibit 99.1 and is incorporated herein by reference.</p>
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<p style="margin:0in 0in .0001pt;text-align:justify;text-indent:.5in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">The information contained in Items 7.01 and 8.01, and in the accompanying exhibit, shall not be incorporated by reference into any filing of the Registrant, 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.&#160; The information in this Current Report, including the exhibit hereto, shall not be deemed to be &#147;filed&#148; for the 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.</font></p>
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<h6 style="font-weight:bold;margin:0in 0in .0001pt;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;">Item 9.01.&#160;&#160;&#160;&#160;&#160;&#160; Financial Statements and Exhibits.</font></b></h6>
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<p style="font-size:12.0pt;margin:0in 0in .0001pt;text-indent:70.55pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">(d)</font><font size="1" style="font-size:8.5pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font>Exhibits</p>
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<p style="font-size:12.0pt;line-height:normal;margin:0in 0in .0001pt 2.0in;text-align:justify;text-indent:-.5in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">99.1</font><font size="1" style="font-size:8.5pt;">&#160;&#160;&#160;&#160;&#160; </font>Press Release dated June 18, 2013 concerning the Registrant&#146;s updated third quarter 2013 outlook and authorization of a $100 million stock repurchase program.</p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">2<a name="PB_2_230618_7056"></a></font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">SIGNATURES</font></b></p>
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<p style="margin:0in 0in .0001pt;text-align:justify;text-indent:1.0in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">Pursuant to the requirements of the Securities Exchange Act of 1934, the&nbsp;registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">TETRA TECH, INC.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">Date:<u> &nbsp;&nbsp;June 18, 2013&nbsp;&nbsp;</u></font></p>    </td>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">By:</font></p>    </td>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">/S/ DAN L. BATRACK</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">Dan L. Batrack</font></p>    </td>   </tr>
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<p style="margin:0in -13.5pt .0001pt 0in;"><font size="3" face="Times New Roman" style="font-size:12.0pt;">Chairman and Chief   Executive Officer</font></p>    </td>   </tr>  </table>
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{"company-data": {"conformed-name": "TETRA TECH INC", "cik": "0000831641", "assigned-sic": "8711", "irs-number": "954148514", "state-of-incorporation": "DE", "fiscal-year-end": "0928"}, "filing-values": {"form-type": "8-K", "act": "34", "file-number": "000-19655", "film-number": "13920509"}, "business-address": {"street1": "3475 EAST FOOTHILL BOULEVARD", "city": "PASADENA", "state": "CA", "zip": "91107", "phone": "6263514664"}, "mail-address": {"street1": "3475 EAST FOOTHILL BOULEVARD", "city": "PASADENA", "state": "CA", "zip": "91107"}}
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):
June 18, 2013
TETRA   TECH, INC.
(Exact name of registrant   as specified in its charter)
Delaware
0-19655
95-4148514
(State or other   jurisdiction
of incorporation)
(Commission
File Number)
(IRS Employer
Identification Number)
3475 East Foothill Boulevard,   Pasadena, California  91107
(Address of principal   executive office, including zip code)
(626) 351-4664
(Registrant's telephone   number, including area code)
Not Applicable
(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:
o
Written   communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)
o
Soliciting   material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)
o
Pre-commencement   communications pursuant to Rule 14d-2(b) under the Exchange Act   (17 CFR 240.14d-2(b))
o
Pre-commencement   communications pursuant to Rule 13e-4(c) under the Exchange Act   (17 CFR 240.13e-4(c))
Item 7.01.       Regulation FD Disclosure.
On June 17, 2013, the Board of Directors of Tetra Tech, Inc. (the "Registrant") authorized the repurchase of up to $100 million of the Registrant's common stock (the "Stock Repurchase Program").  A copy of the press release issued by the Registrant concerning the Stock Repurchase Program and the event described in Item 8.01 below is furnished herewith as Exhibit 99.1 and is incorporated herein by reference in its entirety.
Item 8.01.       Other Events.
On June 18, 2013, the Registrant issued a press release that updated its third quarter 2013 outlook
due to increased restructuring costs resulting from the previously announced weakness in Eastern Canada and mining, and new findings on certain project claims.  A copy of the press release issued by the Registrant concerning the foregoing and the event described in Item 7.01 above is furnished herewith as Exhibit 99.1 and is incorporated herein by reference.
The information contained in Items 7.01 and 8.01, and in the accompanying exhibit, shall not be incorporated by reference into any filing of the Registrant, 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 Current Report, including the exhibit hereto, shall not be deemed to be "filed" for the 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.
Item 9.01.       Financial Statements and Exhibits.
(d)
Exhibits
99.1
Press Release dated June 18, 2013 concerning the Registrant's updated third quarter 2013 outlook and authorization of a $100 million stock repurchase program.
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SIGNATURES
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 hereunto duly authorized.
TETRA TECH, INC.
Date:
June 18, 2013
By:
/S/ DAN L. BATRACK
Dan L. Batrack
Chairman and Chief   Executive Officer
3