wife Susan has delivered a baby girl, reports people.com.
The duo, who married in 2005, already have two-and-a-half year old son Exton.
The 49-year-old star, who has become a father for the third time, also has a son, Indio, from his previous marriage.
Last week, the couple was spotted at the BAFTA Los Angeles Jaguar Britannia Awards where the actor was honoured with the Stanley Kubrick Britannia Award for Excellence in Film.
Girl Joy!! Avri Roel Downey, 7lbs, 20 in., 3:22 a.m. 11-4-14... Pretty like mama, and dimples like my uncle Jim!— Robert Downey Jr (@RobertDowneyJr) November 5, 2014
"It's just the uncertainty of how nowadays you have all these things where you think you have much more of an understanding of what you're going to get, and then this little soul lets herself be known to you, and it's just like: you don't know anything. It's great!" he said.
The Iron Man star received the award from director Jon Favreau and actor Jamie Foxx.
"The second pregnancy has been extraordinarily tough for me. Self-prescribed mandatory bed rest was of some benefit. At least my milk is finally coming in," he said in his acceptance speech.
The proud father first made the announcement of the pregnancy via a humorous Tweet in July. 'Yo. Susan. Me. Baby. Girl. November. Scorpio?' he wrote.