The It's Always Sunny in Philadelphia actor announced the news via his Twitter page on Sunday.
"Check out my beautiful preggers wife, y'all," Howerton wrote on the social networking site, linking to a photo of the couple attending the Cowboys & Aliens premiere in Los Angeles.
The couple will welcome the child later this year.
Howerton's It's Always Sunny in Philadelphia co-stars Charlie Day and Mary Elizabeth Ellis revealed this week that they will also become parents this autumn.
Day recently starred opposite the likes of Jason Bateman, Jason Sudeikis, Colin Farrell and Jennifer Aniston in Seth Gordon's R-rated comedy Horrible Bosses.
Howerton also used Twitter earlier this year to reveal that Fringe star Lance Reddick has signed up for a guest role in It's Always Sunny in Philadelphia.
The show will return for a seventh season this fall on FX.