Harrelson was on CBS' "Late Show with David Letterman" Tuesday night when he told the host his wife, Laura Louie, is pregnant, E! Online reported Wednesday.
"The wife is preggers ... and we've narrowed it down to me," Harrelson said.
Harrelson, 44, and Louie, 40, already have two daughters: Denni Montana, 12, and Zoe Giordano, 9. Harrelson did not reveal a due date nor whether he knew the sex of the latest Harrelson-to-be.