Maybe ... because the YouTube superstar tells TMZ Sports he's 100% down and his promoter Eddie Hearn says it all comes down to getting the money right.
KSI, Hearn and boxing champ Billy Joe Saunders sat down for a special "TMZ Sports: Roundtable" ... and talked about their fighting futures -- and that's when Bieber came up.
"I don't know what weight Justin Bieber can get up to but I think that's a natural fight," Hearn joked.
KSI laughed and told us he's certain he would beat the pop star -- but that's when Hearn started telling us he thinks there's a legit chance the two could end up in the ring together.