A teen was forced to perform a sex act on a knife-wielding man she didn’t know when he cornered her in a city park bathroom in Brooklyn, police said Tuesday.

The 18-year-old told cops she had gotten off the subway around 8 a.m. Monday and was walking when she decided to use the bathroom in Luther Playground, cops said.

Once inside the women’s room, she was confronted by a man who had a knife, she told police.

“He told her to back up and he shut the bathroom door,” a police spokesman said.

The attacker then forced her to perform a sex act, cops said.

The case is being investigated by the Brooklyn South Special Victims Division.