She and boyfriend Matthew Koma welcomed a daughter on Oct. 25, Duff reported on Instagram Monday.
The “Younger” star wrote that she delivered Banks Violet Bair at home and the new arrival “is absolute magic.”
Koma, a musician, described the feeling as “cloud ten” on his Instagram.
Duff, 31, has a 6½-year-old son with ex-husband and former NHL player Mike Comrie.
She announced she was expecting in June, writing that she and Koma “made a little princess of our own.”