Sorry, fight fans -- Floyd Mayweather is "officially done" ... meaning there's not gonna be a rematch with Conor McGregor -- so says his kid.

We spoke to Koraun after the fight ... and he confirmed that Floyd's hangin' up his gloves.

"No chance. No chance. No rematch," Koraun told TMZ Sports, when we asked if he might come back for another HUGE payday.

"Officially done. He's 40, gotta realize that. He's an old man."

Floyd's son says his dad's got a business to take care of too ... so he's happy retiring at 50-0.

FYI, Mayweather said the McGregor fight was his last one "for sure."