An NYPD cop who worked as a lifeguard during his youth put that experience to good use on Sunday and rescued an unhinged woman who jumped into the Hudson River when her boyfriend dumped her, cops said.

Officer Christopher Collins and his partner, Officer Tim Poon, were dispatched to Pier 62 in Manhattan shortly before 8 a.m. in response to reports of a couple engaged in a loud argument, cops said.

The fight was over the man’s intention to end their relationship, and the woman responded by leaping off the pier, the NYPD said.

By the time the cops arrived, the 33-year-old woman had already drifted about 20 feet from the pier.

“I saw the girl right here floating, struggling, she had like a big backpack,” Collins said.

Collins – who said he worked as a lifeguard at a pool in North Bergen, New Jersey, about 20 years ago – quickly grabbed a rescue buoy and stripped off his gun belt, bullet-proof vest and shirt.

“I just jumped in, grabbed her and brought her to the buoy,” he said.

The hero cop said he had to swim about 100 feet to grab the woman and pull her to safety, but downplayed his life-saving effort.

“Any other cop would do the same thing – well, the ones that can swim,” he said.

A 71-year-old man who watched the incident unfold said the woman appeared to have been suicidal.

“At first, I didn’t hear much, but then they started getting louder and the girl said ‘You did this to me’ and the guy said ‘Let’s be adults, we knew this relationship didn’t work,'” said the man, who gave his name as Richard.

“Then she started saying things like she was going to kill herself.”

The woman was apparently wearing the man’s backpack, which he wanted her to hand over, and at one point, she climbed onto a railing and was sitting on it as they argued, he said.

“I’m not sure if she actually jumped or in the struggle she fell over, but she fell into the water and she had the backpack on at the time,” he said.

“She obviously knew how to swim she wasn’t swimming, but she was floating on her back and she kept yelling at him, using some poor words.”

The woman, who wasn’t identified, was taken to Bellevue Hospital for a psychiatric evaluation, the NYPD said.