"Meet Alexis Olympia Ohanian Jr.," the tennis champion wrote on Instagram alongside an adorable mother-daughter photo.
Williams also shared a video on her Instagram story of her pregnancy journey, confirming her daughter's date of birth as Sept. 1. She later posted it on her website and YouTube.
In the video, there are various clips of Williams throughout her pregnancy as well as moments at the hospital pre- and post-birth. At one point, while holding little Alexis, Williams reveals they spent about a week in the hospital.
"It's been a long time," she says. "We had a lot of complications, but look who we got! We got a baby girl!"
Inside the issue of Stellar, Williams opened about her pregnancy, saying, "I think that giving birth will give me more strength... I feel like I'll be ready for anything."