Downey made the announcement Wednesday on social media, sharing the plaudit along with the news that he'll be welcoming a baby girl in November.
"Um. I don't know if it's a 'man's world', but I'm certain women run it," the 49-year-old wrote on Facebook. "Susan and I are therefore delighted to announce we are expecting a baby. Girl. November. rdj"
He also made another version of the announcement on Twitter.
"Yo. Susan. Me. Baby. Girl. November. Scorpio?" he tweeted.
It's the third child for the "Chef" star, who has a son named Exton Elias Downey with Susan, 40. He also has a 20-year-old son, Indio Falconer Downey, from a 14-year marriage to Deborah Falconer that ended in 2004.
Indio made headlines late last month after being arrested on suspicion of drug possession.
The Oscar-nominated actor and his second wife met on the set of his 2003 flick "Gothika," on which she was a producer. They wed on Aug. 27, 2005, and welcomed Exton on Feb. 7, 2012. In addition to many other projects, Susan Downey is a producer on her husband's
Downey joked with Jay Leno in December 2011 that his wife's first pregnancy had been "tougher" on him than on her.
"Just the hormones and the mood stuff and the nausea, and the whole thing," he quipped, also blurting out that they were expecting a boy.