"I am so sad to share this news, but I've been in the hospital recovering from a medical emergency the last couple of days and I won't be able to perform at ONE MusicFest," the Grammy-winning rapper wrote on Instagram. "It breaks my heart that I wont get to see my fans this weekend."
She added, "I'll be back better and stronger soon. Don't Worry."
The 32-year-old New York native gave no details on her condition.
Cardi gave birth to her third child with rapper Offset less than two months ago. The two are going through a divorce.
She was to have performed along with Earth, Wind & Fire, Nelly, Gunna and GloRilla at the two-day ONE Music Fest.