Don't rule out a Justin Bieber vs. Tom Cruise fight in the UFC ... so says Dana White who tells TMZ Sports there's a very REAL chance the two could actually throw down in the Octagon.
After 25-year-old Bieber tagged the UFC president in a twitter post last week calling out 56-year-old Tom Cruise, White says he blew it off initially ... until he got some phone calls from "real guys" connected to the superstars.
Dana wouldn't say who he spoke with but was clear they're legit -- and they told Dana both Tom and Justin "really wanna do this fight and they believe that Tom Cruise would do the fight.