The firefighter was struck by the air conditioner, which apparently fell from the window of a second-story home on 85th Street in the Bensonhurst area sometime after 9 a.m. Monday.
Video of the incident was supplied by residents to firefighters conducting an investigation into the case.
FDNY members rushed to his aide after the firefighter was knocked unconscious.
He was taken to Lutheran Hospital, where he was stabilized. He is expected to survive.
The firefighter's helmet was cracked and sliced open by the AC unit.
Five other firefighters suffered minor injuries from fighting the blaze.
MORE: Video shows owner torching his own restaurant in NY, investigators say