Nate Diaz to Bieber: 'Shut Your Bitch Ass Up'

Justin Bieber is in a brand new celebrity feud ... with UFC superstar Nate Diaz!!!

Here's the deal ... after the fight, Bieber praised Conor McGregor on social media -- posting a photo of the guy with the caption, "Still the champ."

Then he posted video of a toast he made to Conor saying, "To McGregor. He's still the champ. That broke my heart."

He added, "No bandwagon but Conor is a true champion, fights with style and finesse, all the respect to Nate but his style is terrible. Nate has crazy heart respect."

So, how did Nate Diaz react???

He posted a image on Instagram showing him slapping Bieber in the face with the caption, "Shut your bitch ass up."

He added the caption, "Bieber's a hater."

Your move now, Justin ...