The baby boy will join older brother, Axel Lee, who celebrated his first birthday last month.
"We're having a boy! We're so happy that Axel gets to have a brother so close in age. And that I get to add another set of balls to my very, very male world," Olson, 36, said in a statement to the magazine. "We found out I was pregnant during my mom's birthday weekend in Ojai. I was instantly so proud of this baby for already knowing how to make someone else's special day all about him."
The couple's happy baby news is one of many for the FX comedy. Star Glenn Howerton and his wife welcomed their first child last month, and Charlie Day and Mary Elizabeth Ellis are expecting their first child in December. McElhenny, 34, Howerton and Day are also executive producers and writers on It's Always Sunny, which is in the middle of its seventh season and has already been renewed for Seasons 8 and 9.
Olson and McElhenney have been married since 2008.