Floyd Mayweather autograph seekers need to pay VERY close attention ... 'cause the boxing superstar has one specific demand when signing his signature.
Money was at the Manny Pacquiao vs. Keith Thurman fight in Vegas over the weekend ... when a female fan asked him to sign a boxing glove for her.
Floyd grabbed for the glove -- fully prepared to scribble his John Hancock -- but after a quick inspection, he pulled back and told her he wouldn't sign.
And, why not?? He's got a reason .