SOULJA BOY MAYWEATHER'S GOT MY BACK ... In Chris Brown Fight

Soulja Boy is breaking out the big guns in his boxing war with Chris Brown -- he's recruited Floyd Mayweather to help him gear up for the "fight of the century."


After Soulja and Chris challenged each other to a charity boxing match, Soulja met up with the head of The Money Team ... who seems to have Soulja's back.


Soulja says Floyd's gonna promote the bout -- and Floyd insists the two are gonna "get that money, man."


No word if Floyd plans to train Soulja ... but it would be a great move for the rapper since he already turned Bieber into a bruiser.