NorthWestern : 2021 Proxy Statement (PDF)

This proxy statement contains information related to the solicitation of proxies by the Board of Directors (the Board) of NorthWestern Corporation d/b/a NorthWestern Energy (NorthWestern, the company, we, us, or our) in connection with our 2021 Annual Meeting of Shareholders. See the Proxy Statement Glossary on the inside back cover for additional definitions used in the proxy statement.

IMPORTANT VOTING INFORMATION

If you owned shares of NorthWestern Corporation common stock at the close of business on February 22, 2021 (the Record Date), you are entitled to one vote per share upon each matter presented at the annual meeting of shareholders to be held virtually, via the internet, on April 22, 2021. Shareholders whose shares are held in an account at a brokerage firm, bank, or other nominee (i.e., in "street name") will need to obtain a proxy from the broker, bank, or other nominee that holds their shares authorizing them to vote at the annual meeting.

Your broker is not permitted to vote on your behalf on the election of directors and other matters to be considered at this shareholders meeting, except on the ratification of our appointment of Deloitte & Touche LLP as our independent registered public accounting firm for 2021, unless you provide specific instructions by completing and returning the voting instruction form or following the instructions provided to you to vote your shares via telephone or the internet. For your vote to be counted, you will need to communicate your voting decisions to your broker, bank, or other financial institution before the date of the annual meeting.

YOUR VOTE IS IMPORTANT

Your vote is important. Our Board strongly encourages you to exercise your right to vote. Voting early helps ensure that we receive a quorum of shares necessary to hold the annual meeting.

ASSISTANCE

If you have any questions about the proxy voting process, please contact the broker, bank, or other financial institution where you hold your shares. The Securities and Exchange Commission also has a website (www.sec.gov/ spotlight/proxymatters.shtml) with more information about your rights as a shareholder. You also may contact our Investor Relations Department by phone at [phone] or by email [e-mail].

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS

FOR THE ANNUAL MEETING OF STOCKHOLDERS TO BE HELD ON APRIL 22, 2021

The Notice of Annual Meeting, Proxy Statement, and 2020 Annual Report to

Shareholders are available on the internet atwww.proxyvote.com.

ATTENDING THE ANNUAL MEETING VIRTUALLY

We are choosing to hold the Annual Meeting of Shareholders virtually due to the COVID-19 pandemic, taking into consideration the safety of our shareholders and employees. Only shareholders of record or their legal proxy holders as of the record date or our invited guests may participate in the virtual annual meeting. If you wish to participate in the virtual annual meeting and your shares are held in street name at a brokerage firm, bank, or other nominee, you will need the 16-digit control number that can be found on your Annual Meeting Notice.

A replay of the Annual Meeting will be available on our website at NorthWesternEnergy.com under Our Company / Investor Relations / Presentations and Webcasts through April 22, 2022.

Notice of 2021 Annual Meeting and Proxy Statement

March 5, 2021

Dear Fellow NorthWestern Corporation Shareholder:

You are cordially invited to attend the 2021 Virtual Annual Meeting of Shareholders to be held on Thursday, April 22, 2021, at 10:00 a.m. Mountain Daylight Time atwww.virtualshareholdermeeting.com/NWE2021. We're choosing to hold the Annual Meeting of Shareholders virtually due to the COVID-19 pandemic, taking into consideration the safety of our shareholders and employees.

At the meeting, shareholders will be asked to elect the Board of Directors, to ratify the appointment of our independent registered public accounting firm for 2021, to approve our equity compensation plan, to hold an advisory "say-on-pay" vote on the compensation of our named executive officers and to transact any other matters and business as may properly come before the annual meeting or any postponement or adjournment of the annual meeting. The proxy statement included with this letter provides you with information about the annual meeting and the business to be conducted.

YOUR VOTE IS IMPORTANT. We urge you to read this proxy statement carefully. Whether or not you plan to attend the virtual annual meeting online, we encourage you to vote promptly through the internet, by telephone, or by mail.

For details on how to access the virtual annual meeting, please see the information under the caption "Attending the Annual Meeting Virtually" below. If you are unable to attend our virtual annual meeting, a webcast will be available on our website at NorthWesternEnergy.com under Our Company / Investor Relations / Presentations and Webcasts, for one year after the meeting.

Thank you for your continued support of NorthWestern Corporation.

Very truly yours,

Robert C. Rowe

Chief Executive Officer

3010 West 69th Street|Sioux Falls, SD 57108

NorthWesternEnergy.com

Table of Contents

Annual Meeting Notice 1

Proxy Summary 2

Items of Business to Be Considered at the Annual Meeting 6

2020 Executive Pay Overview 2

ESG Sustainability 3

Proposal 1 - Election of Directors 6 Proposal 2 - Ratification of Deloitte & Touche LLP as the Independent Registered Public Accounting

Firm for 2021 8

Proposal 3 - Approve Equity Compensation Plan 10

Proposal 4 - Advisory Vote to Approve Named Executive Officer Compensation 13

Executive Pay 14

Compensation Discussion and Analysis 14

Compensation Committee Report 36

2020 Executive Pay 37

2020 Director Pay 45

Audit Committee Report 46

Corporate Governance 47

Board of Directors 48

Board Diversity 49

Individual Directors 50

Board Independence 55

Board Committees 56

Other Governance Practices 60

Stock Information 62

Who Owns Our Stock 62

Stock for Compensation Plans 64

Annual Meeting Information 65

Voting Procedures 65

General Information 67

Appendix 70

Equity Compensation Plan Summary 70

Equity Compensation Plan 81

Proxy Statement Glossary (inside back cover)