The shooting happened inside the Naciones Barber Shop on Bath Avenue in the Bath Beach section at 1:30 p.m.
Authorities said 25-year-old officer Michael Mininni was getting a haircut when he fatally shot 77-year-old Carmelo Calabro, who had come into the barber shop with a gun, authorities said.
According to investigators, the two men began arguing. There had been some sort of ongoing dispute between their families, sources said.
A struggle ensued, and Michael Mininni shot Calabro in the face. He was pronounced dead at the scene.
"What occurred this afternoon in Brooklyn was a horrible tragedy for the families of all of those involved. However, the Correction Officers' Benevolent Association firmly believes Correction Officer Michael Mininni acted in self-defense when his life was suddenly threatened by an apparent assailant," COBA President Norman Seabrook said.
Mininni has been on the force for a couple of years.
No charges have been filed and the investigation is continuing.