Mariah Carey -- Dating Billionaire James Packer (PHOTO)

Mariah Carey has got a new boyfriend, TMZ has learned, a billionaire who's the 4th wealthiest person down under.

The guy is James Packer, a 47-year-old entertainment mogul who sold a media company and now owns a string of hotels and casinos around Australia and other countries. He's worth an estimated $4.6 billion, according to Forbes.

We got this pic of Mariah and James taken Friday in Capri, Italy. They're staying on Packer's yacht and we're told they'll be on the boat for "an extended vacation." After Capri, it's on to Cannes and St. Tropez.

As for how they met ... Brett Ratner.

Sources close to Mariah tell us James has been aggressively pursuing Mariah for months, and he succeeded.