A crazed man smacked a woman across the face on a subway train in Brooklyn, police said.

The woman, 30, was attacked around 8:30 a.m. Friday on a Manhattan-bound 2 train near the Borough Hall station. She was talking with a group of friends when the suspect unexpectedly struck her on the left side of her face, said cops.

The suspect quickly fled the scene when the train pulled into the station. But a good Samaritan followed the suspect and took his picture.

The man is is described by police as approximately 35 years old. He wore a blue skull cap, a red and white coat and black jeans.