A 24-year-old woman was sexually assaulted at a Manhattan subway station by a stranger who slammed her head down onto the platform – and then ran off with her cellphone when a good samaritan stepped in and thwarted the terrifying attack, police said Monday.

The sicko accosted the victim as she was waiting for a northbound N train at the 14th Street-Union Square station around 3:25 a.m. Sunday and grabbed her breast, cops said.

She tried to get away but wasn’t able to and the creep shoved his hand down her pants and grabbed her private parts before forcing her down on the platform, according to cops.

He then slammed her head to the ground and continued to grope her – before a bystander intervened and scared him off, police said.

The perv ran away, taking the woman’s phone with him, according to cops.

She was later treated at Mount Sinai Morningside for unspecified injuries, police said.

The NYPD released surveillance footage of the suspect in the horrifying attack, showing him passing through the turnstile to leave the station, dressed in dark clothing.

He was described by cops as a man with a dark complexion, believed to be about 40 years old, standing about 6 feet tall and weighing around 160 pounds.