"We haven't told anyone... we're having a boy!" he blurted out when pressed by Meyers.
Kutcher and Kunis are already parents to 2-year-old daughter Wyatt Isabelle. The couple co-starred on That '70s Show in their early careers and married in July 2015 after three years of dating.
"I was actually hoping for a girl because first of all, our first girl is unbelievable, but also, I figured if we had a girl second she might give me one more shot to have a boy and now I don't know," Kutcher said. "She might cut me off! I want, like, twelve of them."
"I don't know. We haven't discussed it yet -- we're not into it -- so this isn't a tabloid argument at all, like the least bit," he clarified. "We're very excited and very happy."
Kunis had said on the Kyle and Jackie O Show in August that she intends to raise her kids without the trappings of celebrity. The actress explained that both she and Kutcher come from "pretty solid poverty backgrounds."
"[We] grew up very poor and are very much self-made and are very aware of what a dollar is worth," she said at the time. "Nothing's been handed to us."
"It's a matter of teaching them from a very early age that 'Mommy and Daddy may have a dollar, but you're poor. You are not rich. You are very poor. You have nothing. Mommy and daddy have a bank account,'" the star added.