An elderly woman is dead after a fire broke out in her apartment on the Upper East Side early Friday, FDNY officials said.

The deadly inferno started around 5:30 a.m. on the third floor of a building on First Avenue between East 91st and East 92nd streets.

The victim, an 86-year-old woman, was found in the bathroom of her apartment and pronounced dead at the scene, fire officials said.

Firefighters hauled out a bare mattress with burn marks from the building early Friday morning.

Boris Yusupov, 40, the owner of Red Chair Studio a few doors down, said the woman was a regular at his shop.

“Makes me feel sad … not because I lost a customer, she was a nice lady,” he said.

Neighbors described the victim as a former teacher and longtime resident of the neighborhood.

She “always devoted herself to school, to teaching kids,” Yusupov said.

A firefighter was treated at a local hospital for injuries.

The fire, which is under investigation, was under control by 6:10 a.m., officials said.

Additional reporting by Gabrielle Fonrouge