The diva is suffering from bronchitis, she told her fans on Instagram Wednesday.
"Hey guys. ... I've been fighting bronchitis for the past few days.. almost better but on mandatory vocal rest per doctor's orders," she wrote. "I'm so sorry to cancel tonight's show but I promise to make it up to you!!! Love you always, MC."
Just last week, Carey kicked off her residency, "Mariah #1 to Infinity," in which she performs all 18 of her number one singles, at Caesars Palace.
"It’s even harder now because as a single mom, it’s not like, ‘Woo, I’m going out tonight to do this and that," the singer said. "I occasionally, on a rare occasion, I will, but it’s much more about watching movies with the kids and having fun and reading them stories or making up stories with them or playing with them when I have free time."