The 49ers didn't have many issues on the field on Thursday night, but their fans in the stands sure as hell did ... getting so mad at each other, they brawled right in their seats!!
The fracas happened at some point during San Francisco's blowout win over the Giants at Levi's Stadium ... when about half a dozen Niner supporters decided to beat the hell out of each other.
It's unclear what started it all -- but both women and men were involved.
You can see in video from the scene, as two ladies decked out in SF gear took swipes at each other ... a man wearing a Richard Sherman jersey grabbed one of the combatants by her hair and threw her several feet down some stairs.
The fight, though, continued ... and when another man jumped into the action, the guy in the Sherman jersey started throwing punches.
Meanwhile, another 49er fan hopped into the fray to land some kicks as well.
No word on how it was all separated ... or if any police were involved. But, if they were ultimately kicked out of the venue for scrapping, they sure missed a great performance from their team -- 'cause the Niners rolled over NY, 30-12.