Cops busted a third teen in the 2016 home invasion and murder of an elderly woman in Brooklyn, police and sources said Wednesday.

Andrew Washington, 19, was arrested around 12:10 p.m. Tuesday following an indictment for the December 2016 murder of 68-year-old Brenda Ramos and her roommate, Joseph Alexander, 40, after breaking into their Kings Highway apartment.

Washington — along with Nycoyan Mattocks and Nia Williams, both 17 — slipped into the pair’s Brownsville apartment and attacked them with a silver hammer, fatally bludgeoning Ramos in the head, police said.

They fled after swiping Alexander’s cellphone, wallet and video game console, sources said at the time.

Ramos, who was known in the neighborhood as “Ma,” was rushed to Brookdale Hospital, but could not be saved.

Mattocks and Williams were nabbed in September 2017 and charged with murder and burglary, cops said.

Washington was indicted at the end of July on charges of murder, robbery, assault and burglary, according to authorities.

Additional reporting by Reuven Fenton