A good Samaritan helped stop a ski mask-wearing assailant who was sexually assaulting a woman in a Brooklyn subway station Monday morning, cops said.

The suspect followed a woman into the bathroom of the 95th Street R station in Bay Ridge at about 10 a.m., punched her in the face several times and started sexually assaulting her, according to police.

The woman screamed for help during the attack, and a good Samaritan opened the door to the restroom and said: “What’s going on in here?”

The attacker shut the door, but the good Samaritan pounded on it until the suspect stopped the attack and ran off.

He was last seen wearing all black and a black ski mask, police said.

The woman suffered bruising to her face and was rushed to a nearby hospital after the attack.