Swae Lee was left bleeding and livid after a fan threw something on stage at a concert he was performing at this weekend ... which riled him up enough to threaten to sue.

The Rae Sremmurd rapper was opening for Wiz Khalifa Sunday night at the Starplex Pavilion in Dallas, when at one point a fan threw an object on stage "like a baseball" that smacked Swae in the face and busted his lip open. It also might've knocked out his tooth, as he indicated himself.

It's unclear what exactly the object was, but some eyewitnesses are saying it was either a phone or a wallet, because it contained an ID inside that Swae was trying to read aloud in getting a name for the perpetrator. He was stopped by people around him, including bandmate Slim Jxmmi who tried to drag him away. It's pretty clear though ... Swae wanted the person's info, and he wanted folks to see his injury.

He asked cameras to zoom in on his face, and they obliged. Needless to say, it looked pretty gruesome. His mic was eventually cut off and he stormed off stage in a rage.

Swae later tweeted about the incident, saying the fan was a girl ... and that the injury was bad enough to keep him off tour for at least a few days.