Willis, 59, and Heming, 35, who are already parents to Mabel Ray, 2, welcomed Evelyn on May 5, reported People magazine.
"Both mother and baby are healthy and doing beautifully. Mabel is delighted to have a new baby sister," reads a statement from the actor's representative.
Heming also took to Instagram to share a snapshot of her husband, with whom she celebrated her fifth anniversary on March 21.
"Mabel loves a ride on the escalator! That might (have) been the 50th time," she tweeted along with the photo.