The 24-year-old actress announced the birth of her son on her website and Twitter feed Thursday.
Duff writes that she and husband Mike Comrie welcomed son Luca Cruz Comrie on Tuesday night. He weighed in at 7 pounds, 6 ounces and is the first child for the couple, who married in 2010.
Duff says she and Comrie "feel like the luckiest parents in the world."
Duff gained fame as the star of Disney's "Lizzie McGuire" and more recently had a recurring role on "Gossip Girl."
Comrie retired as a professional hockey player earlier this year. The 31-year-old most recently played with the Pittsburgh Penguins.