UNITED STATES SECURITIES AND EXCHANGE
COMMISSION

Washington, D.C. 20549



FORM 10-K/A



Amendment No. 1

(Mark One)

| | x | Annual Report pursuant to Section 13 or 15(d) of the Securities Exchange
Act of 1934
| | - | ------------------------------------------------------------------------------------



For the fiscal year ended December
31, 2016



| |  ̈ | Transition Report pursuant to Section 13 or 15(d) of the Securities
Exchange Act of 1934
| | - | ----------------------------------------------------------------------------------------



For the transition period from
_______to_______



Commission File Number 0-53713



OTTER
TAIL CORPORATION

(Exact name of registrant as specified in
its charter)



|MINNESOTA                                                      | 27-0383995                          
|-------------------------------------------------------------- | ------------------------------------
|(State or other jurisdiction of incorporation or organization) | (I.R.S. Employer Identification No.)



|215 SOUTH CASCADE STREET, BOX 496, FERGUS FALLS, MINNESOTA | 56538-0496
|---------------------------------------------------------- | ----------
|(Address of principal executive offices)                   | (Zip Code)



Registrant's telephone number, including area code: 866-410-8780



Securities registered pursuant to Section 12(b) of the Act:



| | Title of each class                      | Name of each exchange on which registered
| | ---------------------------------------- | -----------------------------------------
| | COMMON SHARES, par value $5.00 per share | The NASDAQ Stock Market LLC              



Securities registered pursuant to Section 12(g) of the Act:
None



Indicate by check mark if the registrant
is a well-known seasoned issuer, as defined in Rule 405 of the

Securities
Act. (YesxNo )



Indicate by check mark if the registrant
is not required to file reports pursuant to Section 13 or

Section
15(d) of the Act. (Yes __ Nox)



Indicate
by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange
Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports),
and (2) has been subject to such filing requirements for the past 90 days. (Yes xNo )



Indicate
by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive
Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such
shorter period that the registrant was required to submit and post such files). (Yes xNo )



Indicate
by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein and will not be
contained, to the best of the registrant's knowledge, in definitive proxy or information statements incorporated by reference in
Part III of this Form 10-K or any amendment to this Form 10-K. ̈



Indicate by check mark whether the registrant is a large accelerated
filer, an accelerated filer, a non-accelerated filer or a smaller reporting company. See the definitions of “large accelerated
filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check
one):



| | Large Accelerated Filerx                      | Accelerated Filer ̈        
| | --------------------------------------------- | --------------------------
| | Non-Accelerated Filer ̈                        | Smaller Reporting Company ̈
| | (Do not check if a smaller reporting company) |                           



Indicate by check mark whether the registrant is a shell company
(as defined in Rule 12b-2 of the

Exchange
Act). (Yes __    Nox)



The aggregate market value of common stock held by non-affiliates,
computed by reference to the last sales price on June 30, 2016 was $1,260,418,253.



Indicate the number of shares outstanding of each of the
registrant's classes of common stock, as of the latest practicable date: 39,410,825 Common Shares ($5 par value) as of
February 10, 2017.



Documents Incorporated by Reference:



No
documents are incorporated by reference into this Amendment No. 1 on Form 10-K/A. Certain information required by Part III of the
Form 10-K for the year ended December 31, 2016, filed with the Securities and Exchange Commission on February 22, 2017, has been
incorporated by reference from the 2017 Proxy Statement for the 2017 Annual Meeting.


















EXPLANATORY NOTE



This Amendment No. 1 on Form 10-K/A (this “Amendment”)
amends Otter Tail Corporation’s Annual Report on Form 10-K for the year ended December 31, 2016, which was originally filed
with the Securities and Exchange Commission (the “Commission”) on February 22, 2017 (the “Original Filing”).
Otter Tail Corporation is filing this Amendment for the sole purpose of refiling Exhibits 2-B and 2-C (the “Exhibits”)
to the Original Filing in response to comments received from the staff of the Commission to Otter Tail Corporation’s request
for confidential treatment with respect to certain portions of the Exhibits. The Exhibits filed with this Amendment supersede the
Exhibits as previously filed as Exhibits 2-B and 2-C to the Original Filing. In addition, as required by Rule 12b-15 of the Securities
Exchange Act of 1934, as amended, new certifications by our principal executive officer and principal financial officer are included
herein as exhibits to this Amendment. Accordingly, Item 15 of Part IV of the Original Filing is being amended hereby to reflect
the refiling of the Exhibits and the filing of these new certifications.



This Amendment does not make any other changes to the Original
Filing and does not reflect events occurring after the Original Filing or modify or update any of the information contained therein
in any way other than as expressly described in this Amendment.



PART IV



| | Item 15. | EXHIBITS AND FINANCIAL STATEMENT SCHEDULES
| | -------- | ------------------------------------------



| | (a) | List of documents filed as part of this report:
| | --- | -----------------------------------------------



| | 1. | Financial Statements
| | -- | --------------------



Included in the Original Filing



| | 2. | Financial Statement Schedules
| | -- | -----------------------------



Included in the Original Filing



| | 3. | Exhibits
| | -- | --------



The following Exhibits are filed as part of, or incorporated
by reference into, this report.



|      | Previously Filed   |               
|----- | ------------------ | --------------
|      | File No.           | As Exhibit No.
|2-A   | 8-K filed 7/1/09   | 2.1           
|2-B   |                    |               
|2-C   |                    |               
|3-A   | 8-K filed 7/1/09   | 3.1           
|3-B   | 8-K filed 7/1/09   | 3.2           
|4-A   | 8-K filed 8/23/07  | 4.1           
|4-A-1 | 8-K filed 12/20/07 | 4.3           
|4-A-2 | 8-K filed 9/15/08  | 4.1           



1







|      | Previously Filed  |               
|----- | ----------------- | --------------
|      | File No.          | As Exhibit No.
|4-A-3 | 8-K filed 7/1/09  | 4.2           
|4-B   | 8-K filed 11/2/12 | 4.1           
|4-B-1 | 8-K filed 11/1/13 | 4.1           
|4-B-2 | 8-K filed 11/4/14 | 4.1           
|4-B-3 | 8-K filed 11/3/15 | 4.1           
|4-B-4 | 8-K filed 11/3/16 | 4.1           
|4-C   | 8-K filed 11/2/12 | 4.2           
|4-C-1 | 8-K filed 11/1/13 | 4.2           



2







|       | Previously Filed             |               
|------ | ---------------------------- | --------------
|       | File No.                     | As Exhibit No.
|4-C-2  | 8-K filed 11/4/14            | 4.2           
|4-C-3  | 8-K filed 11/3/15            | 4.2           
|4-C-4  | 8-K filed 11/3/16            | 4.2           
|4-D    | 8-K filed 8/3/11             | 4.1           
|4-E    | 8-K filed 8/16/13            | 4.1           
|4-F    | 8-K filed 2/9/16             | 4.1           
|4-G    | 8-K filed 9/27/16            | 4.1           
|10-A   | 2-55813                      | 5-F           
|10-B   | 2-55813                      | 5-G           
|10-B-1 | 2-62815                      | 5-E-1         
|10-B-2 | 10-K for year ended 12/31/89 | 10-E-3        
|10-B-3 | 10-K for year ended 12/31/90 | 10-E-4        
|10-B-4 | 10-K for year ended 12/31/92 | 10-E-5        
|10-B-5 | 10-K for year ended 12/31/92 | 10-E-6        
|10-B-6 | 10-K for year ended 12/31/93 | 10-E-7        
|10-C   | 10-K for year ended 12/31/89 | 10-F          



3







|       | Previously Filed               |               
|------ | ------------------------------ | --------------
|       | File No.                       | As Exhibit No.
|10-C-1 | 10-K for year ended 12/31/89   | 10-F-1        
|10-C-2 | 10-K for year ended 12/31/91   | 10-F-2        
|10-C-3 | 10-K for year ended 12/31/91   | 10-F-3        
|10-C-4 | 10-K for year ended 12/31/91   | 10-F-4        
|10-C-5 | 10-Q for quarter ended 9/30/03 | 10.1          
|10-C-6 | 10-K for year ended 12/31/92   | 10-F-5        
|10-D   | 10-Q for quarter ended 6/30/15 | 10.3          
|10-E   | 2-61043                        | 5-H           
|10-E-1 | 10-K for year ended 12/31/89   | 10-H-1        
|10-E-2 | 10-K for year ended 12/31/89   | 10-H-2        
|10-E-3 | 10-K for year ended 12/31/89   | 10-H-3        
|10-E-4 | 10-K for year ended 12/31/92   | 10-H-4        
|10-E-5 | 10-Q for quarter ended 9/30/01 | 10-A          
|10-E-6 | 10-Q for quarter ended 9/30/03 | 10.2          
|10-F   | 2-63744                        | 5-I           
|10-F-1 | 10-K for year ended 12/31/92   | 10-I-1        
|10-F-2 | 10-K for year ended 12/31/92   | 10-I-2        
|10-F-3 | 10-K for year ended 12/31/92   | 10-I-3        
|10-F-4 | 10-Q for quarter ended 6/30/93 | 19-A          
|10-F-5 | 10-K for year ended 12/31/01   | 10-I-5        



4







|        | Previously Filed                 |               
|------- | -------------------------------- | --------------
|        | File No.                         | As Exhibit No.
|10-G    | 10-K for year ended 12/31/12     | 10-J          
|10-G-1  | 8-K filed 1/31/14                | 10.1          
|10-G-2  | 8-K filed 3/18/15                | 10.1          
|10-H    | 10-Q for quarter ended 3/31/13   | 10.1          
|10-I    | 10-Q/A for quarter ended 6/30/13 | 10.1          
|10-J-1  | 10-K for year ended 12/31/02     | 10-N-1        
|10-J-1a | 10-K for year ended 12/31/10     | 10-N-1A       
|10-J-1b | 8-K filed 4/17/14                | 10.5          
|10-J-2  | 8-K filed 2/04/05                | 10.1          
|10-J-2a | 10-K for year ended 12/31/06     | 10-N-2a       
|10-J-2b | 10-K for year ended 12/31/10     | 10-N-2B       
|10-J-3  | 10-Q for quarter ended 9/30/11   | 10.1          
|10-J-4  | 10-Q for  quarter ended 9/30/16  | 10.1          
|10-J-5  | 8-K filed 4/13/06                | 10.4          
|10-J-6  | 8-K filed 4/19/12                | 10.2          
|10-J-7  | 8-K filed 4/19/12                | 10.4          
|10-J-8  | 8-K filed 4/13/06                | 10.1          
|10-J-9  | 10-K for year ended 12/31/13     | 10-O-12       
|10-J-10 | 333-195337                       | 4.1           
|10-J-11 | 8-K filed 4/17/14                | 10.1          
|10-J-12 | 8-K filed 4/17/14                | 10.2          
|10-J-13 | 8-K filed 4/17/14                | 10.3          
|10-J-14 | 10-K for year ended 12/31/16     | 10-J-14       



5







|        | Previously Filed               |               
|------- | ------------------------------ | --------------
|        | File No.                       | As Exhibit No.
|10-J-15 | 8-K filed 9/26/14              | 10.1          
|10-J-16 | 8-K filed 2/11/15              | 10.1          
|10-J-17 | 8-K filed 2/11/15              | 10.2          
|10-J-18 | 8-K filed 2/11/15              | 10.3          
|10-J-19 | 8-K filed 2/11/15              | 10.4          
|10-J-20 | 8-K filed 2/11/15              | 10.5          
|10-J-21 | 8-K filed 4/15/15              | 10.2          
|10-K    | 8-K filed 5/11/15              | 1.1           
|10-L-1  | 10-K for year ended 12/31/12   | 10-O-1        
|10-L-2  | 10-K for year ended 12/31/12   | 10-O-2        
|10-M-1  | 10-K for year ended 12/31/10   | 10-Q-3        
|10-M-2  | 10-K for year ended 12/31/10   | 10-Q-4        
|10-M-3  | 10-K for year ended 12/31/11   | 10-Q-5        
|10-M-4  | 10-Q for quarter ended 9/30/14 | 10.3          
|10-M-5  | 10-Q for quarter ended 9/30/14 | 10.6          
|10-M-6  | 10-K for year ended 12/31/15   | 10-R-6        
|10-N    | 8-K filed 4/15/15              | 10.1          
|12.1    | 10-K for year ended 12/31/16   | 12.1          
|21-A    | 10-K for year ended 12/31/16   | 21-A          
|23-A    | 10-K for year ended 12/31/16   | 23-A          
|24-A    | 10-K for year ended 12/31/16   | 24-A          
|31.1    | 10-K for year ended 12/31/16   | 31.1          
|31.2    | 10-K for year ended 12/31/16   | 31.2          
|31.3    |                                |               
|31.4    |                                |               
|32.1    | 10-K for year ended 12/31/16   | 32.1          
|32.2    | 10-K for year ended 12/31/16   | 32.2          



6







|    | Previously Filed             |               
|--- | ---------------------------- | --------------
|    | File No.                     | As Exhibit No.
|101 | 10-K for year ended 12/31/16 | 101           



*Management contract, compensatory plan or arrangement required
to be filed pursuant to Item 601(b)(10)(iii)(A) of Regulation S-K.



**Confidential information has been omitted from this Exhibit
and filed separately with the Securities and Exchange Commission pursuant to a confidential treatment request under Rule 24b-2.



***Schedules and exhibits have been omitted pursuant to Item
601(b)(2) of Regulation S-K. The Company hereby undertakes to furnish copies of any of the omitted schedules and exhibits to the
Securities and Exchange Commission upon request.



Pursuant to Item 601(b)(4)(iii) of Regulation S-K, copies of
certain instruments defining the rights of holders of certain long-term debt of the Company are not filed, and in lieu thereof,
the Company agrees to furnish copies thereof to the Securities and Exchange Commission upon request.



SIGNATURES



Pursuant to the requirements of Section 13 or 15(d) 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.



| | OTTER TAIL CORPORATION
| | ----------------------
| |                       
| | By                    
| |                       
| |                       
| |                       
| |                       
| | Dated:  May 15, 2017  



7







EXHIBIT INDEX



|Exhibit Number |  | Description                                                                                                                                                                                                              
|-------------- |  | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|               |  |                                                                                                                                                                                                                          
|2-B            |  | Asset Purchase Agreement, dated as of November 16, 2016, among Otter Tail Power Company, EDF Renewable Development, Inc., Power Partners Midwest, LLC, EDF-RE US Development, LLC and Merricourt Power Partners, LLC.*/**
|               |  |                                                                                                                                                                                                                          
|2-C            |  | Turnkey Engineering, Procurement and Construction Services Agreement, dated as of November 16, 2016, between Otter Tail Power Company and EDF-RE US Development, LLC.*/**                                                
|               |  |                                                                                                                                                                                                                          
|31.3           |  | Certification of Chief Executive Officer Pursuant to Section 302 of the Sarbanes-Oxley Act of 2002 relating to Form 10-K/A.                                                                                              
|               |  |                                                                                                                                                                                                                          
|31.4           |  | Certification of Chief Financial Officer Pursuant to Section 302 of the Sarbanes-Oxley Act of 2002 relating to Form 10-K/A.                                                                                              



| | * | Confidential information has been omitted from this Exhibit
and filed separately with the Securities and Exchange Commission pursuant to a confidential treatment request under Rule 24b-2.
| | - | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



| | ** | Schedules and exhibits have been omitted pursuant to Item 601(b)(2) of Regulation S-K. The Company hereby undertakes to furnish
copies of any of the omitted schedules and exhibits to the Securities and Exchange Commission upon request.
| | -- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------