The "Hunger Games" star and his wife, literary agent Felicity Blunt, welcomed a son on Jan. 25, his rep told ABC News.
Matteo Oliver Tucci, the couple's first child together, was born in London. He joins half-siblings Camilla, 12, and twins Nicolo and Isabel, 14, Tucci's children from his first marriage.
“I believe he is mine," Tucci joked to People. "We are all thrilled to welcome him to this cold, cruel world."
"We are all thrilled that he is here and healthy," he added more seriously.