"They've been living apart for months," the source says. "He's been staying in hotels."
Despite outward displays of solidarity – the couple spent this summer's Mother's and Father's day together with their 3-year-old twins, Monroe and Moroccan – things between them "have been bad for a while," the source adds.
The pop diva, 45, and the current America's Got Talent host, 33, surprised fans by tying the knot in a secret ceremony in 2008 – just weeks after they began dating.
The couple renewed their vows in 2013 with a fairy-tale-themed bash at California's Disneyland.
All in all, it seemed like they had gotten their happy ending. Last July, Cannon told PEOPLE he supported Carey's sexy Instagrams and wasn't worried about other men admiring his wife.
"We're so secure with who we are," he said. "And Mariah's my dream girl."
Carey was previously married for five years to music mogul Tommy Mottola. They divorced in 1998.
When reached, reps for the couple did not comment.