Police are searching for a man who masturbated in front of other passengers on a Brooklyn subway and then lit a cigarette before getting off the train.

The suspect was caught in the act as he lay across a bench on a southbound Q train as it pulled out of the DeKalb station at about 12:30 a.m. Monday, police said.

After he finished, he stood up and lit a cigarette before walking off the train at the Church Avenue station.

He’s believed to be in his 50s and was last seen wearing gray sweatpants, a blue hooded sweatshirt and black and white sneakers, cops said.

He was still at large Monday night.