Taking to Instagram, the tennis champion wrote: "Meet Alexis Olympia Ohanian Jr., born on September 1, 2017 weighing in at almost 2.8kg."
Named Alexis after her father - and Serena's fiance - Reddit co-founder Alexis Ohanian, the little girl looks relaxed while resting on her mum's chest.
Serena then told fans to view her Instagram Stories where she explained that not all was smooth sailing with her first pregnancy.
The 35-year-old revealed that the family stayed in hospital for six days after Alexis' birth due to "complications."
Serena and Alexis announced their engagement a few weeks before their pregnancy announcement.
The pair has been together for two and a half years.
Congrats to the happy family!