Bieber has challenged Cruise to a fight in the Octagon, the home of UFC fighting.
"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," he posted on social media.
"Who is willing to put on the fight?" Bieber asked, before tagging in the president of the UFC Dana White.
Neither Cruise nor White have responded yet to the bizarre request from Bieber.
And there's been no explicit reason given for why Bieber chose to target Cruise.