A firefighter was seriously injured in a 40-foot fall from a window while battling a five-alarm fire at a Manhattan apartment building Friday, according to the FDNY.

The smoke eater was backing down the stairs of a six-story building that caught fire in Hamilton Heights when he accidentally fell through a fifth-story window and into a well behind the building, officials and sources said.

Sources told The Post that the accident occurred while the firefighter was backing up a hose line.

“He received immediate medical attention at the bottom of the shaft from FDNY medics on the scene and trauma paramedics,” FDNY Commissioner Robert Tucker said in a press conference Friday. “Our paramedics, paramedics, paramedics got to him very quickly.”

“We were blessed today to have a fire department medic on scene as well as the elite rescue medics,” EMS Chief Michael Fields said at the news conference.

The firefighter is listed in serious but stable condition at Harlem Hospital, according to the FDNY.

Firefighters battled a fire that broke out on the second floor of an apartment building on West 145th Street between Broadway and Amsterdam Avenue just after 2:40 p.m., officials said.

FDNY Chief of Fire Operations Kevin Woods said strong winds blowing through the front windows fueled the flames as flames spread to the sixth floor.

More than 200 firefighters intervened in the massive fire.

Five civilians and three other firefighters were also injured, but they are believed to be minors, EMS Chief Michael Field said.

About 10 apartments were affected, displacing up to 40 residents, according to the FDNY.

Additional reporting by Joe Marino