Floyd Mayweather Jr. -- Justin Bieber's My Man Crush!

Floyd Mayweather Jr. says his love for Justin Bieber is unconditional ... after the singer came to the rescue of his children.

Floyd was coming out of a West Hollywood liquor store Monday night and could not stop gushing about his good friend's act of kindness the day before ... when 3 of Mayweather's kids got in a bad car crash.

TMZ broke the story ... Mayweather was on the BET Awards red carpet when he got word his kids were in a car wreck. Floyd happened to be talking to Justin on the phone ... and Biebs raced to the scene to comfort the kids.

The boxing champ is taking some heat for not leaving the red carpet and rushing to the aid of his brood.