"When I climbed the Capitol steps in January, I promised myself that I would return to the Senate with an open mind and greater respect for others," Kirk wrote.
"Same-sex couples should have the right to civil marriage. Our time on this Earth is limited, I know that better than most. Life comes down to who you love and who loves you back -- government has no place in the middle."
Kirk joins Rob Portman, R-Ohio, as the second Republican to back same-sex marriage.
Celebrity Breakups and divorces of 2014 [PHOTOS]