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 * Szurek Paul E. 2. Issuer Name and Ticker or Trading Symbol CoreSite Realty Corp [ COR ] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable)__X__ Director                    _____ 10% Owner
__X__ Officer (give title below)    _____ Other (specify below)
President & CEO
(Last)         (First)         (Middle)
C/O CORESITE REALTY CORPORATION, 1001 17TH STREET, SUITE 500
3. Date of Earliest Transaction (MM/DD/YYYY)
3/15/2021
(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  3/15/2021    A    17010 (1) A $0.00  205220  D   
Common Stock  3/15/2021    A    44651 (2) A $0.00  249871  D   
Common Stock  3/15/2021    S(3)    7524 (4) D $117.74  242347  D   

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)  Represents restricted stock granted under the issuer's Amended and Restated 2010 Equity Incentive Award Plan (the "2010 Plan"). The restricted stock vests in three equal annual installments beginning on March 15, 2022, provided that the reporting person remains in continuous service with the issuer as of each vesting date.
(2)  Represents performance-based restricted shares ("PSAs") granted under the 2010 Plan. The number of PSAs earned is based on the issuer's achievement of relative total shareholder return ("TSR") measured versus the MSCI US REIT Index over a three-year performance period ending December 31, 2023, and the number of shares earned may range from 25% to 175%. The PSAs are earned as follows: (i) 20% of the PSAs are earned upon TSR achievement in year one of the performance period, (ii) 20% of the PSAs are earned upon TSR achievement in year two of the performance period, (iii) 20% of the PSAs are earned upon TSR achievement in year three of the performance period, and (iv) 40% of the PSAs are earned upon a cumulative TSR achievement over the three-year performance period, subject to the reporting person's continued service with the issuer at the end of the three-year performance period. The PSAs are granted and issued at 175% of the target amount.
(3)  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.
(4)  Represents shares sold by the reporting person to cover tax obligations upon the vesting of restricted stock.

Reporting Owners
Reporting Owner Name / Address
Relationships
Director 10% Owner Officer Other
Szurek Paul E.
C/O CORESITE REALTY CORPORATION
1001 17TH STREET, SUITE 500
DENVER, CO 80202
X
President & CEO

Signatures
/s/ Derek S. McCandless, Attorney-in-Fact 3/17/2021
**Signature of Reporting Person Date
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