The daughter of Bill and Hillary Clinton shared the news on Twitter with a photo of her 15-month-old daughter Charlotte.
Charlotte was pictured looking down at a book with the title "big sisters are the best".
"Next summer, Charlotte is going to be a big sister. Feeling very blessed and grateful this holiday season," Clinton wrote.
Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season. pic.twitter.com/gpCGqcmeCq— Chelsea Clinton (@ChelseaClinton) December 21, 2015
Shortly after announcing the news, Hillary and Bill Clinton both tweeted their joy at having a second grandchild arriving next year.
Former president Bill wrote: "Christmas comes early! @HillaryClinton and I are thrilled for Chelsea, Marc and Charlotte’s growing family in 2016!"
— Bill Clinton (@billclinton) December 21, 2015
Hillary tweeted: "Your dad and I could not be happier for you, Marc, and Charlotte. We’re so excited to meet our second grandchild!"
Your dad and I could not be happier for you, Marc, and Charlotte. We’re so excited to meet our second grandchild! -H https://t.co/hgTr07HBlr— Hillary Clinton (@HillaryClinton) December 21, 2015
Chelsea hinted earlier in the year that she was keen to expand her family.
The 35-year-old mum-of-one told PEOPLE in September: "I think my love for Charlotte has just opened more space in my heart and my brain that I didn’t even know was there."
Speaking about how motherhood has changed her, she added: "I didn’t know I could care more intensely about anything until I became a mum.
"Somehow I love my husband even more, I love my parents even more."