San Diego Chargers Ryan Mathews -- NOT ARRESTED

San Diego Chargers running back Ryan Mathews was NOT arrested for a bar brawl ... TMZ has learned.

There are multiple reports on the Internet that Matthews knocked out a guy Sunday during a bar brawl in San Diego's Gaslamp district -- but San Diego PD spokesman Edward Zwibel tells us Ryan was not taken into custody, and is not a suspect in the fight.

Zwibel confirms officers did respond for the fight ... and they did make one arrest, but Mathews had nothing to do with it.

In fact, he says officers never even saw Mathews ... though he was aware of rumors that he was in the bar.

So ... star athlete NOT busted. It does happen.