"20 weeks," she captioned the image on Snapchat Wednesday after posting a series of photos from what looked to be a tropical getaway.
US Open organizers wished the star a big congratulations in a celebratory tweet shortly after she revealed the news.
"Serena Williams will have a new pride & joy to hug and call her own soon! Congratulations on the exciting baby announcement!" the post read.
The tennis star shared the photo on the same day her biggest rival, Maria Sharapova, celebrated her 30th birthday.
Williams, 35, is engaged to Reddit co-founder Alexis Ohanian, 33. The US Open champion documented her whirlwind engagement on Reddit back in December with a poem.
Ohanian popped the question during an impromptu trip to Italy. He proposed at a restaurant in Rome at the same table where the duo first met on a lunch outing in 2015.
"This time he made it not by chance / But by choice / Down on one knee / He said 4 words And / isaidyes," she wrote at the time.
Ohanian, a Brooklyn native, also posted to his website at the time of the proposal with the line, "she said yes."
Williams and the techie began dating in October 2015, but didn't discuss their relationship publicly until the engagement news bombshell.
The star was a few weeks pregnant when she beat her sister Venus to win the Australian Open back in January.
Fans were quick to praise the athlete for her supserstar abilties and were mind-blown to learn she competed while a few weeks pregnant.
"Serena Williams won the Australian Open while she was pregnant. That woman is amazing," one Twitter user wrote.
Another added, "Serena Williams was pregnant when she won the Australia Open in Jan. but I cant even get out of bed when I'm on my period. Sis is superwoman."
Williams has won more Grand Slam titles than she is weeks pregnant, reaching the 23rd win early on in her pregnancy.
The pro-athlete has four Gold Medals, one from winning the singles at the 2012 London Olympics and doubles wins at the 2000 Sydney Olympics, 2008 Beijing Olympics and the 2012 London Olympics.
The Florida-native has been ranked No. 1 by the Women's Tennis Association in the singles on seven occasions between 2002 and 2017.
In 2016, she was named the Best Female Tennis Player at the ESPY Awards and in 2015 she was named Sports Illustrated Sportsperson of the Year.
Social media users were quick to slam the idea she was only the best "female" athlete — noting she should be regarded as the best athlete of all time.
"Why Serena Williams has to be called greatest female athlete? I think she's earned the right to drop the female. One the best ever (sic). Period," a Twitter user wrote.
On Monday, Williams shared an adorable snap of her fiancé holding her up in his arms on a beach.
Before finding hapiness with Ohanian, Williams dated rapper Common on and off for about two years before splitting in 2010 and was also rumored to be dating tennis coach Patrick Mouratoglou, NBA star Amar'e Stoudemire and rapper Drake.
Williams and Ohanian have not yet revealed a wedding date, but it’s sure to be a star-studded spectacle.
Beyoncé has been a longtime fan and friend of Williams, even attending her matches at last year's US Open with hubby Jay Z.