"Mae James Bair- We LOVE you beauty 3-24-21," Duff, 33, captioned a photo of her cradling the newborn while sitting in a home birth tub with her 2-year-old daughter Banks Violet.
Looking on from outside the tub are Duff's 9-year-old son Luca Cruz and her husband Matthew Koma.
The family portrait got almost 2 million "likes" since it was posted on Saturday.
On Friday, Duff shared a photo of Banks in a bathtub.
It was captioned, "I'm a big sister ....... marinating on how I feel about that!"
Duff did not reveal the sex, name or birthday of her new baby until a day later, however.