FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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(Print or Type Responses)

1. Name and Address of Reporting Person * Finnin Jeffrey S   2. Issuer Name and Ticker or Trading Symbol CoreSite Realty Corp [COR] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner __X__ Officer (give title below) _____ Other (specify below) CFO and Treasurer (Last) (First) (Middle) C/O CORESITE REALTY CORPORATION, 1001 17TH STREET, SUITE 500 3. Date of Earliest Transaction (Month/Day/Year) 03/22/2017 (Street) DENVER, CO 80202 4. If Amendment, Date Original Filed(Month/Day/Year) 6. Individual or Joint/Group Filing(Check Applicable Line) _X_ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 03/22/2017   M(1)   22,066 A $ 32.4 133,993 D   Common Stock 03/22/2017   S(1)   3,724 (2) D $ 87.57 (3) 130,269 D   Common Stock 03/22/2017   S(1)   10,576 (2) D $ 88.46 (4) 119,693 D   Common Stock 03/22/2017   S(1)   600 (2) D $ 89.27 (5) 119,093 D

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Stock Option (Right to Buy)

$ 32.4

03/22/2017

M

22,066

(6)

02/28/2023

Common Stock

22,066

$ 0

0

D

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Finnin Jeffrey S C/O CORESITE REALTY CORPORATION 1001 17TH STREET, SUITE 500 DENVER, CO 80202			CFO and Treasurer

Signatures

/s/ Derek S. McCandless, Attorney-in-Fact		03/24/2017
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	The option exercise and sales were effected pursuant to a sales plan adopted by the reporting person and intended to comply with Rule 10b5-1 under the Securities Exchange Act of 1934.
(2)	Represents shares sold by the reporting person to cover the exercise price and tax obligations upon the exercise of the options.
(3)	The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $87.11 to $88.09, inclusive. The reporting person undertakes to provide to the issuer, any shareholder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes 3, 4 and 5.
(4)	The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $88.12 to $89.11, inclusive.
(5)	The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $89.13 to $89.46, inclusive.
(6)	The stock options were granted under the issuer's 2010 Equity Incentive Award Plan and vest and become exercisable in four equal annual installments beginning on February 28, 2014, provided that the reporting person remains employed by the issuer as of each vesting date.

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