Devin Haney threw a punch in a brawl at a Southern California fair this week ... and now the incident is the center of an assault and battery report, TMZ Sports has learned.
Check out the video filmed by a patron at the OC Fair in SoCal ... where 25-year-old Haney, who looks to be with a group of friends, fires a left straight at a man who'd seemingly been arguing with the group.
It's unclear if Haney's punch landed, but the fight was on, with at least three others letting the fists fly at the lone man as he attempted to defend himself.
A few seconds later, as the fight continued, sirens could be heard ... which seemed to prompt Haney to walk away from the scene, followed shortly after by the rest of his group.
The Orange County Sheriff's Dept. responded to the scene, according to the agency ... where they met with someone who claimed to be a "victim of an assault and battery."
Aside from the alleged victim, OC says everyone else had already fled the scene ... so no arrests were made.
The law enforcement agency is currently investigating.
The outing marks one of the first times we've seen Haney, 31-0 (the loss to Garcia was overturned), since his scrap with Ryan Garcia in April.
Speaking of Garcia, he chimed in on the fair incident on X ... as the video made its way around the internet, writing, "He couldn’t even drop a civilian."
"Also," the boxer added, "you talked about being a great role model. You really punched someone while your whole team jumps one guy sounds pretty diddy to me."