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

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

YC 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.

YC's rep Kali Bowyer says 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.