U-T San Diego reported that Keiser had an altercation with another man. It escalated into a fight in the city's Gaslamp Quarter, Officer Jim Johnson told the newspaper.
"We're aware of the issue involving Thomas," the Chargers said Monday night in a statement. "We'll continue to monitor the situation and let the legal process run its course."
Keiser, an undrafted free agent out of Stanford, started in three games for the Chargers this season. He has tallied 17 tackles, 4.5 sacks and one interception on the season.
Keiser's next court appearance is set for Feb. 10.