Rapper Stunna Girl -- who's got a solid online following -- performed in Syracuse, New York Friday ... when video shows a fan reaching up and smacking her on the ass.

While the rapper seems to brush it off in the moment, she walks over to a man -- who many fans have identified as her husband RichMula500 -- and whispers something to him ... and, that's when all hell breaks loose.

Another clip shows RichMula now in the crowd ... basically nose-to-nose with the guy he believes touched Stunna's booty. The video of the moment itself doesn't show the fan, so we can't be certain this is the person who touched Stunna -- but RM500 seems pretty confident it is.

The two exchange words ... but, Mula doesn't throw the first punch. Instead, another person onstage comes and grabs the dude by the back of the neck -- before wrestling him to the ground where they start throwing punches at him.

Stunna comes down to grab her husband ... but, she's ushered away to safety -- a good move too because chairs start getting thrown and the whole concert venue erupts into chaos.

If you don't know Stunna, she's up-and-coming in the rap game ... hanging out with the likes of French Montana.

Stunna's accumulated more than half a million monthly listeners on Spotify -- so she's a big deal.