Sugar Ray Leonard -- I Could've Beaten Mayweather ... In My Prime

He beat the father ... but Sugar Ray Leonard says when it comes to the Mayweather family, he could've whooped the son, too.

Sugar Ray was leaving Boa Steakhouse in Hollywood last night when TMZ Sports asked the legendary fighter if he would have beaten Floyd Mayweather Jr. back in his prime.

Leonard is clearly confident.

There's some history though -- back in 1978, Sugar put a hurtin' on Floyd Mayweather Sr., earning a TKO in the 10th round of their fight ... sending Leonard's record to 14-0.

Leonard was a beast back in the day -- there's no question he would've given Junior a run for his money.