"I wanna challenge Tom Cruise to fight in the octagon. Tom if you dont take this fight your scared and you will never live it down. Who is willing to put on the fight?" Bieber, 25, tweeted Sunday night.
So far, Cruise, 56, has not publicly responded to the invitation.
But Smith, 48, has thoughts.
"You wanna control Cruise in this match? 1) Don't fight him on a couch: he can be bouncy. 2) Make sure you're fighting the actual Tom Cruise, because he's king of impossible rubber mask reveals. 3) If he slides into the Octagon in Wayfarers and undies, go for his parents' Porsche," Smith posted on his Twitter account.
No word on what prompted Bieber's challenge.