The 25-year-old singer took to Twitter in June to challenge the 57-year-old Hollywood actor to a fist fight - but Justin now admits it was a ridiculous idea.
Speaking to James Corden on 'Carpool Karaoke', Justin confessed: "I don't know [what I was doing]. I was just being stupid, to be honest.
"[But] then people were like, 'I wanna see this happen.' And I was like, 'You know what? That could actually be funny.'"
James subsequently suggested the proposed fight wouldn't have been "much fun" for Justin.
The TV star explained: "I gotta say, if you and me have a fight, you win. If you and Tom Cruise have a fight, I'd back Tom every day of the week."
But Justin didn't agree with James' assessment of the proposed fight.
The 'Yummy' hitmaker replied: "Absolutely not. I'm trying not to let my ego get in the way, but there's absolutely no way. He's not the guy you see in movies."
James then reiterated his faith in Tom, hailing the actor's fitness levels.
In response, Justin quipped: "I start to get frustrated. You're really boiling me up! I'm gonna f**kin' fight you, bro!"
Justin also likened himself to UFC star Conor McGregor, suggesting he's one of the toughest men in the entertainment business.
The pop star - who married model Hailey Bieber in 2018 - joked: "I'm dangerous! My agility is insane ... I don't think you understand the mind control that I have!
"My mind control is another specimen. I'm different. I'm the Conner McGregor of entertainment!"
In his original tweet, the 'Love Yourself' singer gave no reason for wanting to fight Tom, but said he wanted to take him on in "the octagon".
He wrote: "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? @danawhite ? (sic)"