A member of Mayor de Blasio’s security detail was shot with a BB gun outside Gracie Mansion Sunday night, police said.

The plainclothes detective, who was hit in the buttocks while getting out of her car at about 7:15 p.m., was treated and released from Lenox Hill Hospital, and a suspect in the apparent prank was in custody, police said.

The suspected shooter, identified by law-enforcement sources as Mike Verbitsky, 20, was believed to have been taking shots out a window of his apartment building at 170 East End Ave., across from the mayor’s official residence.

Verbitsky is a known troublemaker, according to his neighbors.

Cops quickly entered Verbitsky’s building, where they questioned his girlfriend, Gabrielle Gleyberman, 19, inside a 10th-floor apartment, sources said.

While Gleyberman was distracting police, Verbitsky sneaked out a separate door and fled downstairs, according to law-enforcement sources.

He was apprehended outside by cops.

Both were taken into custody and were being grilled at the 19th Precinct station house Sunday night.

Meanwhile, police were conducting a thorough search of the residence, sources said.