Lewis actor Fox confirmed on Twitter that wife Piper had given birth to a boy.
"One born every minute," he wrote. "We had our minute today. Couldn't be happier. Beautiful boy. Everything fine."
Piper announced that she was pregnant again last October, with a report in January revealing the couple were expecting a boy.
However, the pair are yet to name the newborn. Piper and Fox, who married in 2007, are already parents to 3-year-old Winston.
A friend told the Daily Mail: "Mother and baby are doing extremely well. Billie and Laurence are thrilled to bits."