That's a fired-up Conor McGregor making some serious threats to Justin Gaethje -- telling the UFC's interim lightweight champ that he's coming for Gaethje's head.

"Your teeth. I’m going to put them on a f**king necklace," Conor said to Justin.

In an angry Twitter rant Monday, McGregor says he wants to fight Justin next -- and it seems the hostility comes from a 2019 tweet Justin sent criticizing Conor as a "sh*t human, father and husband."

Conor remembered the tweet and responded ...

"Speak on my skills as a father? You are f**king dead."

McGregor added, "Don’t ever say you represent the great nation of the United States of America ever again."

For his part, Justin has been calling for a fight with Conor for a while -- telling TMZ Sports back in 2019 he actively tried to get Conor to sign a contract but the Irishman wouldn't do it.

"I wrote him a message like f**king 7 months ago just trying to be his friend. I’ve tried every route," Justin said.

"I’m trying reverse psychology. I’m trying to be his friend. I’m trying every f**king route that there is to draw this man out. And I can't draw him out. And he’s calling out Frankie Edgar. So, it’s a slap in the face to me, to all the lightweights, and to really the standard that we f**king do here."

Of course, Justin beat the hell out of Tony Ferguson at UFC 249 on Saturday ... and says he now wants to fight Khabib Nurmagomedov for the real lightweight title.

But, a Conor fight could be a bigger payday for Justin ... so it seems like he's got some decisions to make.