Party of four!
The couple's new bundle of joy joins older sister, Wyatt Isabelle, who was born in October 2014. No word yet on the baby boy's name.
WATCH: Find Out What Ashton Kutcher Wanted to Name His Baby Boy (Before Mila Kunis 'Knocked Down' The Idea)
Kutcher, 38, accidentally let the gender of baby No. 2 slip during an appearance on the Today show in early October.
"She points to Mila and she's like, 'baby brother' when she points to the belly," he said, referring to their daughter's reaction to Kunis' growing bump. "And then she points to dad and she goes, 'Beer,' and so I think she gets there's something in the belly, but I don't quite think that she knows that it's not going to be a plastic doll yet."
He later confirmed they were having a boy during a sit-down with Seth Meyers.
"We haven't told anyone. We're having a boy!" Kutcher said on Late Night. "I was hoping for a girl because first of all, our first girl is unbelievable. But also I figured if we had a girl second, [Mila] might give me one more shot at having a boy and now I don't know. It's even money, she might cut me off. I want, like, 12 of them."
WATCH: Mila Kunis Flaunts Baby Bump Weeks Before Due Date -- See the Pic!
ET caught up with Kunis, 33, after it was revealed she was expecting again and she explained the differences with her pregnancy this time around.
"I forgot how far along I was today," she told ET's Kevin Frazier at a special screening of her comedy, Bad Moms, in July. "I think with the first one, you're so enamored with the initial experience that I was constantly checking the app, 'What fruit size is the baby? Is it the size of a grape or an avocado?'"
"Now I'm like, 'Wait, how far along am I?'" she added. "You're busy running after the other one and working full time."
Kunis and Kutcher, who co-starred together on the sitcom That '70s Show, quietly tied the knot in July 2015.
For more on the Hollywood pair, watch below: