A Halloween party turned bloody in Brooklyn on Sunday when a gunman “starting letting rounds off,” killing a man and wounding an apparently innocent female bystander, cops said.

The gunfire rang out around 6:30 a.m. in a rented party space at 1006 Atlantic Ave in Prospect Heights, leaving the fatal victim shot in the back and the woman hit in the elbow, police said.

The 51-year-old woman was believed to be an innocent bystander, cops said.

Police were still investigating whether the dead man was the intended target.

“She just happened to be there,” a detective said of the injured woman. “An unknown person started letting rounds off.”

The woman was taken to Kings County Hospital.