Yep, the biggest stars in the UFC are well aware of the possible Tom Cruise vs. Justin Bieber superfight ... and top-ranked welterweight Ben Askren is already picking a winner.

"I'm gonna say Tom Cruise," Askren says ... "Like, Tom Cruise at least seems like he's probably been through some sh*t."

As we previously reported, Bieber's rep, Scooter Braun, recently told Dana White he'd be down to fight FOR REAL if Tom signs on ... and Dana says it would be the biggest Pay-Per-View fight of all time.

Ben agrees ... "That would make a whole bunch of money!"

Meantime, Askren has his fight to worry about ... he's throwing down with Jorge Masvidal at UFC 239 on July 6.

But, if 56-year-old Tom Cruise does agree to fight 25-year-old Bieber, Askren says he'd be down to train the 'Mission Impossible' star ... and he's already got a game plan for him.

"All I'd teach him is [the double leg takedown] and then beat his ass ... little typewriter action on his face!"