Nate leveled the allegation in a fiery Facebook post ... saying, "Gsp cheated when he fought my brother he was on steroids."
FYI, GSP defeated Nick by unanimous decision while defending the UFC welterweight title at UFC 158.
Diaz continued ... "He was overweight and they let him be because we were in Montreal (GSP's home turf) and his boy gave nick a bad iv."
Now, with Dana White saying he's trying to make a fight between Nate and GSP ... Diaz has issued an ultimatum.
"Gsp you can clear that sh*t up or fight yourself cheater."
Nick Diaz made the GSP steroid allegations back in 2015 -- claiming the Quebec Athletic Commission never properly tested the guys before the fight because if they had, Nick admits he would have been popped for marijuana.
For his part, GSP has never tested positive for PEDs in his career.