A source close to the couple told Us Weekly that the pair named the baby Charlie Wes.
"They are thrilled … [and the] baby is healthy," the insider said.
Charlie is the second child for Arquette, who already has a 9-year-old daughter, Coco, with ex-wife Courteney Cox.
"I’ve done it once," Arquette told Us Weekly about being a father. "It’s all new, but it’s all beautiful."
The Scream actor and McLarty, an entertainment correspondent, have been dating on and off since 2011. The pair announced they were expecting their fist child in November.