10:43 AM PT -- A rep for the Costa Mesa Police Department tells TMZ Sports cops have launched an investigation into the fight.
Pittsburgh Steelers star WR Chase Claypool was involved in a late-night bar fight in Southern California … and TMZ Sports has the footage.
The 22-year-old -- who just wrapped up a sensational rookie campaign for the Steelers -- was at the Wild Goose Tavern in Costa Mesa, CA with some friends on March 13 when things popped off with another group of patrons.
Sources tell us there was an issue involving someone throwing dollar bills at a woman inside the bar, which sparked an argument … and eventually, things moved outside.
We’re told Chase was NOT an instigator -- but as you can see in the video, things started to get out of control and Chase jumped into the action to help out one of his buddies.
At one point, the 6’4”, 238-pound NFL star gets in the middle of the action -- where one of the combatants was wrestling on the ground -- and Claypool delivers a swift kick to the face.
Eventually, Chase’s pals remove him from the fracas -- while people yelled at him to stand back and keep his distance.
We’re told both parties ultimately left the scene without further incident.
The Costa Mesa Police confirm they received a call for a fight at that location but when officers arrived, security told them the parties involved had already left. Nobody has been arrested or charged with a crime.
Unclear if the guy who got kicked plans on taking action -- but people close to Chase tell us he was just trying to protect his friend in a chaotic situation, like anyone would do.
We reached out to Claypool for comment, but so far, nothing back.
Claypool was a 2nd-round pick in the 2020 NFL Draft after a stellar career at Notre Dame.
He played in all 16 games for the Steelers as a rookie -- logging 62 catches for 873 yards and 11 total TDs.