The actress gave birth to a baby boy on Wednesday, Nov. 30, her rep confirmed to ABC News.
The former "That 70's Show" co-stars are already parents to a two-year-old daughter, Wyatt.
Kunis, 33, and Kutcher, 38, met almost 20 years ago on the show, where their characters, Kelso and Jackie, were in a relationship. Kunis said she and Kutcher were just friends at the time.
"I was 14 and he was 19. It wasn’t even a possibility. I don’t know how to explain it," Kunis told ABC News earlier this year. "I didn’t realize how good looking he was until we reconnected when I was in my late 20s."
Meanwhile, Kutcher has raved about his wife and mother of his now two children.
"The most amazing thing about having a baby is my partner, Mila. She's the greatest mom," he told Ellen DeGeneres in 2014, after welcoming his daughter.
"I look at this as the greatest opportunity of my life," he added about fatherhood.