Kirby, who joined Chelsea in 2015, won the WSL title six times and the FA Cup on five occasions with the London side, scoring 107 goals in 192 appearances.
"To leave with the trophies that I have won, with the teammates I've played with - some of the world's best players - it has been one of the biggest privileges of my life," the 30-year-old Kirby said in a statement.
WSL champions Chelsea are second in the standings with 46 points from 19 games, six behind leaders Manchester City who have played one more match.
