The couple has named their new son, Exton Elias Downey, and he weighed in at a healthy 7lb 5oz.
Star magazine first broke the news that the couple was expecting, via RadarOnline.com back in August.
Downey already has one son, Indio, 18, from his 12-year marriage to Deborah Falconer.
As RadarOnline.com previously reported, Downey announced that the couple was expecting a boy last December.
The actor also talked about the trials of pregnancy, quipping to Jay Leno: “It’s been tougher on me. Just the hormones and the moods stuff and the nausea, and the whole thing.”