A vigilante posing as an NYPD detective tried to stop a fare-evading family in Lower Manhattan this week — and even pepper-sprayed a man who didn’t comply, cops said. 

The imposter flashed a detective shield he wore around his neck as he confronted a family attempting to enter the Whitehall Street-South Ferry R train station through an open exit gate around 6:35 p.m. Sunday, police said. 

“I’m a detective with NYPD,” the suspect declared, according to police. 

A 46-year-old man with the family group tried to walk away from the fake cop — who then unloaded some pepper spray at the back of his head, authorities said. 

The vigilante then boarded a northbound R train, taking it a few stops to 8th Street-NYU, where he was seen on surveillance cameras, cops said. 

The victim, meanwhile, was taken by private means to Bellevue Hospital, where he was treated and released.

The suspect, identified by police as 35-year-old Dominic Ogrady, was last seen wearing ripped denim shorts, a black and yellow T-shirt, a yellow hat, and an NYPD Detective Shield bearing the number 694, hanging from a chain around his neck.

Anyone with information on the crime is asked to call the NYPD’s Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).

The public can also submit their tips by logging onto the Crime Stoppers website at https://crimestoppers.nypdonline.org/, on X @NYPDTips.