Zuckerberg announced the happy news on Facebook on Thursday, alongside sweet throwback photos of him and Chan growing up with their own sisters.
"Priscilla and I are happy to share we're expecting another baby girl!" he began his post. "After our difficult experience having Max, we weren't sure what to expect or whether we'd be able to have another child. When Priscilla and I first found out she was pregnant again, our first hope was that the child would be healthy. My next hope was that it would be a girl. I cannot think of a greater gift than having a sister and I'm so happy Max and our new child will have each other."
"We are all better people because of the the strong women in our lives -- sisters, mothers and friends," he concluded. "We can't wait to welcome our new little one and do our best to raise another strong woman."
Zuckerberg and Chan started dating in 2003 and married in 2012. The pair announced they were expecting their first child, daughter Max, in July 2015, in an emotional post revealing their struggles to have children.