The victim, identified as Nichelle Thomas, 51, was shot just before 1 p.m. at Fourth Avenue and St. Marks Place in Park Slope.
Footage obtained by The Post shows Thomas approaching a bodega at the corner of St. Marks Place — apparently unaware her ex is about to launch an attack.
As Thomas goes to open the door, 38-year-old Latisha Bell allegedly sprints up and pulls a gun. Bell then allegedly shoots Thomas point-blank in the back of the head and flees.
The chilling nine-second video ends with the victim crumpling to the ground.
She was rushed to a hospital, where she died, police said.
The two women used to be in a relationship, police sources said. It wasn’t immediately clear when they broke up.
Police didn’t immediately provide a motive for the shooting. But investigators are looking into the possibility that the fatal encounter was sparked by a lovers’ quarrel, according to law enforcement sources.