Sources close to the couple tell TMZ ... Mariah's planning on getting hitched this summer on the island where her soon-to-be husband is currently building a private $250 million resort with Robert De Niro.
She's only inviting a small group of friends, who will be flown in on private jets ... naturally.
No word on the exact date, but it'll likely be sometime after mid-June. She's been on a comeback after landing new management -- a sold-out Vegas stint, a new 10-part docuseries, and her first European tour in a decade ... the gig runs through May and just kicked off to rave reviews and loads of screaming fans.
We're told invitations haven't gone out yet, so Mimi could change her mind. When you're richer than God and marrying a billionaire ... everything's possible.