Mr Ohanian posted a photo of the 23-time Grand Slam winner smiling while cradling the couple’s second daughter in her arms, alongside their eldest child Alexis Olympia Ohanian Jr, aged five.
Mr Ohanian wrote: “Welcome, Adira River Ohanian.
“I’m grateful to report our house is teaming with love: a happy & healthy newborn girl and happy & healthy mama. Feeling grateful.
“Serena Williams you’ve now given me another incomparable gift — you’re the GMOAT. Thanks to all the amazing medical staff who took care of my wife & our daughter.
“I’ll never forget the moment I introduced Olympia Ohanian to her baby sister.”
In a second photo Mr Ohanian is seen placing the baby on Olympia’s lap.
Earlier this month Williams revealed she and her husband were expecting a baby girl with a drone display.
The tennis champ, 41, shared a six-minute long video to YouTube which documented her preparation for her “baby shower slash gender reveal” party.
Before learning she was having another girl, the athlete admitted she was “Team Pink”, adding that she was “unprepared” if it was to be revealed that she was having a son.
The couple tied the knot in November 2017, just a few months after welcoming Olympia in September 2017.
Williams announced she was having her second child back in May when she showed off her blossoming bump at the Met Gala.
She later confirmed the news in a post shared to her Instagram, writing, “Was so excited when Anna Wintour invited the 3 of us to the Met Gala.”
When Williams announced her retirement from professional tennis in Vogue last August, she wrote about how Olympia had expressed that she wanted to be a big sister.
“I’m the youngest of five sisters myself, and my sisters are my heroes, so this has felt like a moment I need to listen very carefully to,” Williams wrote in Vogue.