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

August 19, 2013

Date of Report (Date of earliest event reported)

NORTHWEST NATURAL GAS COMPANY

(Exact name of registrant as specified in its charter)

Commission File No. 1-15973

220 N.W. Second Avenue, Portland, Oregon 97209

(Address of principal executive offices) (Zip Code)

Registrant’s Telephone Number, including area code: (503) 226-4211

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 8.01 Other Events.

On August 19, 2013, Northwest Natural Gas Company (NW Natural) issued and sold $50,000,000 aggregate principal amount of its 3.542% Secured Medium-Term Notes due 2023 (the Notes) pursuant to the terms of a Distribution Agreement, dated March 18, 2009, as amended by the Company’s Notice dated August 19, 2011, among NW Natural and Banc of America Securities LLC, UBS Securities LLC, J.P. Morgan Securities LLC., Piper Jaffray & Co., Wells Fargo Securities, LLC, U.S. Bancorp Investments, Inc., Mitsubishi UFJ Securities (USA), Inc., RBC Capital Markets, LLC, CIBC World Markets Corp., and TD Securities (USA) LLC. The Notes are registered under NW Natural’s automatic shelf registration statement on Form S-3 (SEC File No. 333-171596) (the Registration Statement).

In connection with the issuance and sale of the Notes, NW Natural is filing two legal opinions regarding the validity of the Notes as Exhibit 5(a) and 5(b) to this Form 8-K for the purpose of incorporating the opinions into the Registration Statement.

Forward-Looking Statements

This report, and other presentations made by NW Natural from time to time, may contain forward-looking statements within the meaning of the U.S. Private Securities Litigation Reform Act of 1995. Forward-looking statements can be identified by words such as “anticipates,” “intends,” “plans,” “seeks,” “believes,” “estimates,” “expects” and similar references to future periods. Examples of forward-looking statements include, but are not limited to, statements regarding the following: plans, objectives, goals, strategies, future events, investments, customer growth, weather, commodity and other costs, customer rates, revenues and earnings, dividends, performance, effects of future regulatory proceedings, and other statements that are other than statements of historical facts.

Forward-looking statements are based on our current expectations and assumptions regarding our business, the economy and other future conditions. Because forward-looking statements relate to the future, they are subject to inherent uncertainties, risks and changes in circumstances that are difficult to predict. Our actual results may differ materially from those contemplated by the forward-looking statements. We caution you therefore against relying on any of these forward-looking statements. They are neither statements of historical fact nor guarantees or assurances of future performance. Important factors that could cause actual results to differ materially from those in the forward-looking statements are discussed by reference to the factors described in Part I, Item 1A “Risk Factors”, and Part II, Item 7 and Item 7A “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and “Quantitative and Qualitative Disclosure about Market Risk” in the Company’s most recent Annual Report on Form 10-K and in Part I, Items 2 and 3 “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and “Quantitative and Qualitative Disclosures About Market Risk”, and Part II, Item 1A, “Risk Factors”, in the Company’s quarterly reports filed thereafter.

All forward-looking statements made in this report and all subsequent forward-looking statements, whether written or oral and whether made by or on behalf of the Company, are expressly qualified by these cautionary statements. Any forward-looking statement speaks only as of the date on which such statement is made, and we undertake no obligation to publicly

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update any forward-looking statement, whether as a result of new information, future developments or otherwise, except as may be required by law. New factors emerge from time to time and it is not possible for the Company to predict all such factors, nor can it assess the impact of each such factor or the extent to which any factor, or combination of factors, may cause results to differ materially from those contained in any forward-looking statements.

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Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

The following exhibits are being filed pursuant to Item 8.01 herein.

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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.

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EXHIBIT INDEX

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