Here's a first ... Floyd Mayweather is using the Forbes list as diss fuel -- saying Ronda Rousey is still way too poor to be mentioned in the same breath as the boxing champ.
Obviously, Floyd caught wind of Ronda's anti-Mayweather video Tuesday -- where she called him illiterate and bragged that she makes more money per second than he does.
So, Floyd's doing what boxers do ... hitting back.
In a statement issued through a TMT member, Mayweather tells TMZ Sports ... "Big bank take little bank. Let's play and see how much money you got."
He added, "Last time I checked I was #1 on Forbes. She will never be at my status. Keep trying to get publicity."
And you thought Meek Mill vs. Drake was bad ...