The tennis great announced Wednesday that she and fiancé Alexis Ohanian are expecting their first child "this fall."
Williams, 35, posted a side profile shot of herself in a yellow swimsuit on Snapchat with a caption that read "20 weeks" before deleting it.
A spokesperson for the tennis player later confirmed the American was pregnant.
If Williams' 20-weeks post was accurate, it would suggest she was about eight weeks pregnant when she won the Australian Open in January.
While Williams will not play again in 2017, meaning she will miss the year's remaining three grand slams -- the French Open, Wimbledon and US Open -- her publicist did say she would return to the tour in 2018.
Williams has not played since winning her 23rd grand slam in Melbourne, withdrawing from the BNP Paribas Open at Indian Wells, California, and the Miami Open citing a knee injury.
Williams, who is second only to Margaret Court on the list of women's all-time grand slam singles titles, has received congratulatory messages from around the world.
Former world No.1 Andy Roddick tweeted: "There's gonna be a baby GOAT... We are so happy and we know @serenawilliams will be a great mother."
Christ Evert, 18-time grand slam winner, said it was a "very special year ahead" for Williams.
Steve Simon, Women's Tennis Association (WTA) chief executive, said: "We're excited for Serena. This is wonderful news and we wish her the very best. We'll look forward to seeing her when she's back competing again."
Williams will not be the first player to return to the tennis tour after giving birth.
Belgium's Kim Clijsters triumphed at the US Open in 2009 to become only the third mother to win a grand slam, following in the footsteps of fellow tennis moms Court and Evonne Goolagong.
Former world No.1 Victoria Azarenka is also set to make her comeback in July after giving birth to her first child in December.
Williams' latest news comes after she surprised fans in December by announcing her engagement to Ohanian, Reddit's co-founder. The pair had kept their romance out of the spotlight.
She had alerted the world of her impending nuptials on Reddit, with tech entrepreneur Ohanian posting on his Facebook page, linking to her Reddit post and adding: "She said yes."