A woman descending a flight of stairs in an upper Manhattan apartment building died in a freak accident Thursday evening — slipping out a window by the staircase and plunging to her death, cops said.

The tragedy happened at about 8:30 p.m. as the unidentified woman was on a staircase in an Inwood apartment building.

She somehow slipped between the sixth and fifth floors and tumbled out an open window.

First responders found her at the rear of the building and declared her dead.

Initial reports said she may have jumped, but cops say they believe she accidentally fell.