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
                                                                                  
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940
                      
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)
Chief Financial Officer
(Last)         (First)         (Middle)
C/O CORESITE REALTY CORPORATION, 1001 17TH STREET, SUITE 500
3. Date of Earliest Transaction (MM/DD/YYYY)
5/3/2021
(Street)
DENVER, CO 80202
(City)       (State)       (Zip)
4. If Amendment, Date Original Filed (MM/DD/YYYY)
 
6. Individual or Joint/Group Filing (Check Applicable Line) _X _ Form filed by One Reporting Person
___ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)
2. Trans. Date 2A. Deemed Execution Date, if any 3. Trans. 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  5/3/2021    S(1)    1158  D $121.06 (2) 70516  D   
Common Stock  5/3/2021    S(1)    2842  D $122.00 (3) 67674  D   
Common Stock                 5000  I  By family trust (4)

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 3)
2. Conversion or Exercise Price of Derivative Security 3. Trans. Date 3A. Deemed Execution Date, if any 4. Trans. 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 7. Title and Amount of Securities Underlying Derivative Security
(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

Explanation of Responses:
(1)  The shares were sold 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)  The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $120.76 to $121.43, 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 2 and 3.
(3)  The price reported in Column 4 is a weighted average price. The shares were sold in multiple transactions at prices ranging from $121.77 to $122.59, inclusive.
(4)  The reporting person's spouse is the trustee of the family trust.

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


Chief Financial Officer

Signatures
/s/ Derek S. McCandless, Attorney-in-Fact 5/5/2021
**Signature of Reporting Person Date