Rapper YC -- Busted Cheating in Vegas ... Fiancee Ditches Him at the Altar!

EXCLUSIVE

Rapper YC -- aka Yung Chris -- just learned fidelity is a key part of marriage ... especially the night before your wedding.

Sources close to YC say the rapper and his GF of 5 years, Tia Anderson ... decided to elope at the end of March, and invited a handful of family and friends to join them at the Bellagio in Vegas. What they didn't plan on was the smokin' hot bartender.

We're told Tia spotted YC -- best known for his hit, "Racks" -- getting handsy with a hotel bartender on Friday night ... and immediately pulled the plug on the Saturday ceremony.

YC's telling friends nothing happened with the drink-slinger, but admits it was heading in that direction. Not that it matters to Tia ... the damage was done.

As for the ring -- we're told Tia's keeping it. If YC's looking for ice ... try the bartender.