Stewart, who was recently elected a member of the Class of 2020 of the NASCAR Hall of Fame, competed in an All-Star Circuit of Champions winged sprint car event last Friday night at Jackson (Minn.) Motorplex – a series that Stewart owns.
Stewart, however, blew an engine at the start of the main feature and ended up credited with last place.
In the garage and pit area after the race, Stewart was continuously heckled by a fan. At first, he ignored the man, but after he persisted with more insults, Stewart took off after the guy, followed him around a pickup truck and appeared to throw a punch at his head.
The two were then separated by other fans that were present.
A video of the incident has been posted on YouTube.
(Warning: Contains profane language and violent behavior)
Following the incident, Stewart returned to signing autographs for fans. Reports say he was seen icing his hand.
On Sunday night, Stewart drove to victory at 34 Raceway in West Burlington, Iowa, where he held off dirt stars Rico Abreu and Christopher Bell to take the win.
When first contacted about the incident by Motorsport.com, Stewart-Haas Racing officials said they had no official comment and were looking into what transpired.
Stewart, 48, retired from fulltime NASCAR competition following the 2016 season. During his career, he won three Cup series championships, including 49 Cup series races. He is also a former IndyCar Series champion.