The tennis star often posts photos to social media of her family life - but there won't be any party pictures from Alexis Olympia's special day.
Williams was asked about how she would be celebrating her daughter's birthday at a press conference at the US Open.
She replied: "Olympia doesn’t celebrate birthdays. We’re Jehovah’s Witnesses, so we don’t do that."
Jehovah's Witnesses do not celebrate religious holidays such as Christmas and Easter, and do not celebrate birthdays either. They believe that these 'festivals' have pagan origins.