Facinelli, 48, best known for his Twilight films, and Harrison, 33, recently starred in The Vanished.
In an Instagram post, Facinelli announced the birth. Their child's sex has not yet been revealed.
Facinelli wrote "Happy 'Labor' Day, @lilyanneharrison," alongside a picture of his newborn. "Sept. 5, 2022.
Facinelli has three previous children from his ex-wife Jennie Garth: Luca, 25, Lola, 19, and Fiona, 15.
In June, Harrison announced her pregnancy on Instagram.
"Not a burrito belly," Harrison captioned the post.
In the months that followed, Harrison shared a number of behind-the-scenes shots.
The pregnancy has been easy for her. "It was a pleasant surprise because every pregnancy is different."
So I kind of didn't know what was going to happen, you never know going in."
During a Mexican vacation at the end of 2019, Facinelli and Harrison became engaged.