The couple already share a son, Dakota.
The couple quietly welcomed a second son, Carson, around Christmas last year, Us Weekly reports.
The former Disney actress and the Home Alone star's new bundle of joy joins his older brother, Dakota, who was born in April 2021. Dakota was named in honor of Culkin’s sister, who died in 2008.
Culkin, 42, and Song, 34, worked together on the set of Changeland in Thailand, and have been together for nearly four years now. Culkin first spoke about his desire to have kids with the former Disney star someday in an interview on The Joe Rogan Experience podcast back in August 2018.
"I'm gonna make some babies," Culkin said, when asked if he wants children. "This one's a good one, so I'm probably going to put some babies in her in a little bit. I mean, we've definitely been practicing."
"This one, I'm going to have some pretty babies. She's Asian, so I'm gonna have tiny, little Asian babies. It's going to be adorable -- a bunch of Sean Lennons running around the house, that's what I'm looking for," he added, referencing the son of the late John Lennon and Yoko Ono.
The baby news comes one year after the longtime couple got engaged to be married.