Floyd Mayweather: Justin Bieber Ain't No Bitch ... He Fought with Heart!

Floyd Mayweather LOVES the way Justin Bieber fought Wednesday night -- telling TMZ Sports, "He showed he ain't no bitch."

Bieber's been training with the boxing legend for years -- but Mayweather says the most impressive part of the fight video isn't JB's form ... "He showed he got heart."

Mayweather says he doesn't know why Bieber got into it in Cleveland last night -- but says the lesson is clear ... "Justin Bieber ain't no bitch."