Nate Diaz is firing shots at Conor McGregor and Dana White -- mocking the UFC duo on social media while proclaiming HIMSELF the real champ. 

"There's 1 boss here," Diaz wrote under a picture of him busting out the Stockton Slap on both guys.

That's not all ... Nate also ripped into the photo Conor and Dana took together on Sunday after landing in L.A. for their Mayweather fight media tour.

"Little bitches," Diaz wrote ... "Got your ass beat. You lost bitch. Im the champ..."

Diaz hasn't stepped inside the Octagon since losing to McGregor in their rematch at UFC 202 back in August 2016. 

White has told TMZ Sports he TRIED to make deals with Nate and his bro, Nick Diaz, to book them in other events ... but they flat-out refused every offer the UFC made. 

Neither Dana nor Conor have responded to Diaz's comments -- yet -- but with the Mayweather media tour launching Tuesday, you can bet they'll have some thoughts on this dude. 

IT MIGHT BE OVER FOR HIM
TMZSports.com