The 42-year-old actor's representative confirmed the happy news, saying the baby arrived on Monday, April 28 in Los Angeles.
"Mom, dad and baby are doing well. They appreciate all of your well wishes and thank you for respecting their privacy at this time," Arquette's representative said.
West joins big sister Coco who is Arquette's nine-year-old daughter with ex-wife Courteney Cox.
Arquette and entertainment correspondent McLarty have been dating on and off since 2011, briefly splitting in 2012.