A heroic FDNY paramedic plunged into freezing waters to rescue a man in the Hudson on Saturday afternoon.

Niall O’Shaughnessy, 40, a veteran paramedic with the department, was working with his partner Minze Wu along the West Side Highway around 2:50 p.m. when he noticed some NYPD units going into the river at Harrison and West Streets — and realized they might need help.

“I grabbed my personal flotation device and ran over to where the NYPD was,” O’Shaughnessy recalled.

He kicked off his boots and removed his radio before jumping in.

The NYPD had already lowered a life preserver to the man, but the helpless victim was unable to hold it, the paramedic said.

“I used my body to pull him around the life ring,” O’Shaughnessy said, adding he spent about 5 to 10 minutes in the icy water.

“The first two seconds in, I felt the cold set in,” he said. “After about 30 seconds to a minute I could definitely feel that my arms and legs weren’t working.”

The man had started to lose consciousness, O’Shaughnessy said.

“He was definitely lethargic,” the paramedic said. “He was definitely hypothermic. He had difficulty speaking and holding onto the ring. I was behind him holding onto him on the life ring.”

Fast-responding firefighters lowered a ladder to O’Shaughnessy to bring him and the 50-year-old victim back up to land.

The man was taken to Bellevue in stable condition. It was unclear why he was in the water.

O’Shaughnessy was briefly evaluated at New York-Presbyterian Downtown Hospital.

Despite saving the man’s life, O’Shaughnessy said he wouldn’t consider himself a hero.

“I was just a civilian today,” he said. “I wouldn’t say I was a rescuer. He just needed a little help.”

The gutsy first responder is no stranger to the grimy Hudson River. Back in July 2015, he took the plunge to save a woman who had jumped into the waters.

And a year earlier, he hopped onto the subway tracks at the 49th Street Station in Midtown to save a drunken Irish woman who fell in between a pair of train cars.

O’Shaughnessy, too, is originally from Ireland. His hometown is Limerick, which about 2 hours away from Dublin.