The 45-year-old songstress has been involved with the 'Rush Hour' director for weeks since her split from Nick Cannon, reported Aceshowbiz.
Ratner was the director of Carey's eight music videos including "Heartbreaker", "I Still Believe" and "We Belong Together."
"Their relationship has progressed in recent weeks and is now very serious. They're a hot couple," a source said.
The insider said the rumoured couple jetted off to St Barts on Australian billionaire James Packer's private jet on Friday, March 27 for a joint birthday.
Carey's birthday was on that day while Ratner, 46, was on the next day.