A 53-year-old man walking past the USS Intrepid dived into the chilly Hudson River yesterday and rescued a 15-year-old girl who had just jumped in.

The good Samaritan saw the girl leap into the river near Pier 84 at 2:40 p.m. and immediately jumped in after her, witnesses said.

Cops said a rescue crew found them sitting atop a piling under the pier, at 12th Avenue near 44th Street.

An FDNY boat ferried the girl to shore, and an ambulance took her to a hospital for evaluation.

As for her hero, who was uninjured, he left without giving his name.