The 34-year-old actor made the announcement in a video for his ALS Ice bucket challenge, reported Us magazine.
"I'd like to nominate Baby O'Porter," the 34-year-old said, before the camera pans to a heavily pregnant O'Porter.
She then pours a small glass of water on her baby bump, to which O'Dowd said, "So brave!"
The couple have been married since 2012 and indicated that they wanted to start a family last year.
Hayden Panettiere recently used the challenge to also announce her pregnancy.