A Washington Football Team fan unloaded on a fellow fan in the stands during the team's game against the Chargers on Sunday ... dropping haymaker after haymaker on the poor guy.
Unclear what started the tilt ... but the man in the Portis jersey quickly got the upper hand -- and never let off the gas pedal until other fans finally intervened.
The clip shows the dude threw roughly 13 punches -- with many of them landing flush on the other man's face.
No word if anyone suffered injuries in the fight ... but we're betting at the very least, they're both pretty damn sore today.
As for the action on the field, the WFT ended up losing to L.A., 20-16 ... so, yeah, tough day all around for these guys.
FYI, this was far from the only fight at an NFL game Sunday ... Rams and Bears fans also got into a wild brawl at SoFi Stadium during L.A.'s game against Chicago.