Floyd Diaz, an undefeated boxer, was arrested in Sin City last week … after Las Vegas Metro cops pulled the pugilist over while driving a stolen vehicle, TMZ Sports has learned.

During the stop, LVMPD also found the 21-year-old boxer was carrying a firearm he was not legally allowed to possess.

Diaz was initially stopped after a patrol unit received a LoJack alert indicating the vehicle was stolen.

The fighter was booked into the Clark County jail on two charges … possession of a stolen vehicle and carrying a concealed weapon without a permit. Bail was set at $10K total.

The charges, both class C felonies, carry up to 5 years in prison, in addition to hefty fines.

Diaz, 13-0 bantamweight, is a young up-and-coming fighter born and raised in Vegas. He last fought in September, beating Mario Hernandez at Madison Square Garden by unanimous decision.