Justin Gaethje got congratulations messages from all kinds of people following his big win over Dustin Poirier ... but maybe the most notable to call was Donald Trump, who actually sent an invitation to the UFC star to golf with him as part of his well-wishes chat.
Gaethje said the former POTUS gave him a ring shortly after his stunning KO victory on Saturday night ... telling TMZ Sports the call was "awesome."
"He f***ing loves fighting, man," Gaethje said of No. 45.