Lewis Gordon Pugh took to the freezing waters on Sunday to highlight the devastating impact of climate change on the natural world.
It took him 18 minutes and 50 seconds to swim 0.6 miles in waters created by melted sea ice at temperatures of 29°F — the coldest a human has swum in.
"I am obviously ecstatic to have succeeded but this swim is a triumph and a tragedy," the 37-year-old British lawyer said after coming out of the water.
"A triumph that I could swim in such ferocious conditions, but a tragedy that it's possible to swim at the North Pole."
Pugh said he hoped that his swim will make world leaders take climate change seriously.
"The decisions which they make over the next few years will determine the biodiversity of our world," he said.
"I want my children, and their children, to know that polar bears are still living in the Arctic — these creatures are on the front line up here."
Swimming has given him a unique perspective on climate change, Pugh says on his Web site.
"I have witnessed retreating glaciers, decreasing sea ice, coral bleaching, severe droughts and the migration of animals to colder climates.
"It's as a result of these experiences that I am determined to do my bit to raise awareness about the fragility of our environment and to encourage everyone to take action."
Calling it the hardest swim of his life, Pugh said Sunday that the water was black when he jumped in.
"It was like jumping into a dark black hole. It was frightening. The pain was immediate and felt like my body was on fire," said Pugh, who's an ambassador for the World Wildlife Fund UK.
"I was in excruciating pain from beginning to end and I nearly quit on a few occasions."
Colin Butfield of WWF UK called the challenge "a bittersweet victory, as this swim has only been possible because of climate change."
Pugh is known for his epic swims in waters from the Antarctic to the Indian Ocean.
His ability to raise his body temperature in anticipation of a swim in cold waters has intrigued top sports scientists and has earned him his "Polar Bear" nickname.