Robert Downey Jr. "couldn't be happier" about the birth of his second son.

The actor's wife Susan gave birth to Exton Elias Downey this morning at 7.24am, People magazine has confirmed.

His rep told the publication that the baby boy weighed in at 7lbs, 5oz and is 20 inches long.

"Everyone is healthy and they couldn't be happier," a friend of the couple told the publication.

Baby Exton joins big brother Indio, who is Robert's 18-year-old son from his previous marriage.

Robert announced the sex of the baby last year during a television interview which Susan was not too happy about.

"I could never really stay mad at him," she said at the time. "But I can tell you it genuinely wasn't something we planned to reveal."

The Sherlock Holmes: A Game of Shadows star joked that if he had also revealed the baby's name before the birth that Susan "would lop my head with a machete."