Floyd Mayweather -- Got My Butt Kicked ... On The Basketball Court (VIDEO)

Floyd Mayweather finally took an L ... courtesy of an NBA star who beat the boxer in a "Best of 5" pickup basketball game ... though Floyd definitely held his own.

Floyd went up against Boston Celtics PG, Isaiah Thomas, on the hardwood ... and even though they're close in height -- Floyd is 5'8" and Thomas is 5'9" -- Isaiah was playing on another level.

But we gotta admit ... in the footage featured on Showtime's "All Access" and pointed out by our friends at BSO, Mayweather flashed some skill ... and even sank a nice little teardrop.

Hey, there's always next time ...