This Seattle Seahawks fan had a painful Saturday night -- because after his team lost to the Cowboys, he tried to fight a bunch of Dallas fans at AT&T Stadium ... and lost.

The person who shot the footage tells TMZ Sports ... the Hawks fan was runnin' his mouth to Dallas fans after the game and finally, one of the locals had enough.

You can see in the footage, the Seattle fan was throwin' haymakers -- but never really connected. Turns out, having long hair in a street fight can really work against ya.

The Hawks' fan's friend tried to help his buddy out -- but like the Seahawks, it just wasn't his night.

We're told event security eventually got into the middle of the action and broke it up -- unclear if anyone suffered any major injuries.