A teen gunman opened fire inside a Long Island gym during a fight with another teen late Tuesday, cops said. 

Noah Haynes, 18, was arguing with a 17-year-old victim inside LA Fitness on Deer Park Avenue in North Babylon around 7:15 p.m. when staff members asked them to leave, Suffolk County police said. 

The pair went out to the parking lot, where the fight turned physical, cops said. 

Haynes then took off in a car, and the younger teen returned to the gym, police said. 

But about 30 minutes later, Haynes returned and confronted the 17-year-old in the locker room, authorities said. 

There, he allegedly slashed the teen in the neck with a metal object and pointed a handgun at him.

Both teens left the locker room and began to fight in the gym area, as the victim yelled to other patrons that Haynes had a gun, cops said.

Three gym members intervened and held Haynes down until police arrived — but he fired one shot during the struggle, police said. 

No one was hit, authorities said. 

Several witnesses called 911 around 7:50 p.m. to report an active shooter, cops said. 

Responding cops arrested Haynes about five minutes later. 

He was charged with second-degree assault, first-degree reckless endangerment, menacing and second-degree criminal possession of a weapon, authorities said.

He was held overnight at Suffolk County’s First Precinct and is scheduled for arraignment at First District Court in Central Islip on Wednesday.

The teen victim was taken to a local hospital, where he was treated for minor injuries and released.