11:30 PM PST -- According to reports, the unruly fan is a corporal with the Tulsa County Sheriff's Office ... the division has said it is opening an investigation regarding the incident. Tony Stewart doesn't take guff from anyone, especially on his turf, as one racing fan learned.
The shoving match went down Friday night at The Lucas Oil Chili Bowl Nationals in Oklahoma. Stewart reportedly approached a fan who had given him the finger and screamed unkind words. It sounds like the guy tells Tony, "I never liked you."
You see the fan push Stewart and then security steps in and takes care of business.
Stewart's in the throes of a wrongful death lawsuit ... the result of his car striking and killing driver Kevin Ward Jr. during a race in 2014. The 2 men had beef but Stewart has been adamant ... the collision was unintentional.