Here's UFC star Mackenzie Dern blowing off some steam on a boat in Mexico -- dancing her ass off with a pretty famous Brazilian singer!

Hey, it's Cinco de Mayo -- why not enjoy yourself, right?!

Mackenzie is coming off a dominant victory over Nina Nunes at "UFC on ABC" back in April ... extending her win streak to 4 in a row!

The 28-year-old is the #5 ranked UFC strawweight becoming a legitimate title threat ... with sites set on challenging Rose Namajunas for the belt one day.

But, for now, she's earned the right to party -- and she's doing just that on a boat in Cancun with her friend Lexa!

FYI, 26-year-old Lexa is pretty huge in Brazil -- she even had her own reality show on MTV a few years back.

Mackenzie is married to Brazilian surfer Wesley Santos. They have one daughter together. But, neither Wes nor the kid seem to be on this trip, which appears to be a fun girls' getaway!