Damon was born on Thursday, January 4, reports People magazine.
The couple is already parents to two-year-old son Conrad.
Reid and Elspeth announced they were expecting by revealing Keller's baby bump in August at the L.A. premiere of the actor's film, 'Home Again'.
The two tied the knot in an intimate ceremony at downtown Los Angeles's Maguire Gardens in June 2014. (ANI)