The Meta CEO, 38, and the philanthropist, 37, are expecting their third baby, another daughter, Zuckerberg announced on social media Wednesday.
"Lots of love. Happy to share that Max and August are getting a new baby sister next year!" he wrote alongside a sweet photo of the couple.
In the photo, Zuckerberg places his hand on Chan's stomach as the pair smile for the camera.
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Zuckerberg and Chan's love story began in 2003 when they started dating after meeting in line for the bathroom at a party at Zuckerberg's Harvard University fraternity when he was a sophomore and she was a freshman.
The couple moved in together in September 2010, he wrote on Facebook at the time. Then, in May 2012, they tied the knot in a ceremony in their Palo Alto backyard — surprising their guests, who thought they were there to celebrate Chan's graduation from UCSF medical school.
Since then, the couple launched the Chan Zuckerberg Initiative together in 2015, pledging 99 percent of their wealth to causes including improving education and curing disease.
The pair recently celebrated their 10th wedding anniversary, commemorating the special milestone by recreating one of the photos they took together on their special day back in 2012.
"10 years married and half our lives together," Zuckerberg wrote alongside the recreated snap. "Here's to more adventures."