The 'Leftovers' actress, who already has son Milo, 10, with ex-husband Royston Langdon, gave birth to her second son on Februray 11, reported People magazine.
"He's healthy and everyone is very happy," a representative for the couple said.
The 37-year-old star confirmed she was expecting a baby in December.
Gardner, who is best friends with David Beckham, has a seven-year-old son called Grey with his ex-wife Davinia Taylor.