Bears often roam thousands of kilometres in a year in search of prey such as seals, and there has often been anecdotal evidence of prodigious ursine swims, with bears turning up on remote islands or across wide bays.
However, there had been doubts about whether the bears walked over ice part of the way or hitched a ride on an iceberg.
"What's new this time is that we have data showing how long the bear was in the water," Jon Aars, a researcher at the Norwegian Polar Institute, said on Friday.
"This is the first time that such a long swim has been documented by satellite telemetry for polar bears," the institute added.
The female bear, equipped with a satellite tracking device, entered the water on the east of the Norwegian Arctic island of Spitsbergen early on 20 July, swam northeast and re-emerged on the island of Edgeoya a day later.
Scientists warn global warming
could threaten polar bear cubs
A sensor on the bear's collar sent different signals when it was in salty sea water compared with on land or on ice.
"This is an astonishing swim," Aars said, saying it showed that polar bears could in many ways be classified as marine mammals - a group including whales and dolphins.
Aars said the bear, dubbed "Skadi" after a Norse goddess of snow, had probably swum closer to 100km since the bear almost certainly did not swim the 74km between the two points in an exact straight line.
The bear covered the gap in about 24 hours, giving an average speed of 3-4kmh, about as fast as a person walking.
"If sea ice retreats from denning areas it will first become a problem for females with small cubs"
Tonje Folkestad of the WWF
The swim probably meant that two cubs, with Skadi when the bear was marked in the spring, had died earlier in the summer.
Mortality rates among polar bear cubs are high.
"We don't think cubs could swim that far - they lose heat much faster than adults," Aars said. Cubs usually stay with their mother for about 2.5 years.
The WWF environmental group said cubs were most at risk in a warming Arctic that could destroy cubs' dens.
"If sea ice retreats from denning areas, it will first become a problem for females with small cubs," said Tonje Folkestad of the WWF.
An eight-nation report by 250 experts last year said the Arctic was warming twice as fast as the rest of the globe and that a buildup of heat-trapping gases from factories, power plants and cars was largely to blame.
It said global warming could make the Arctic Ocean ice-free by the end of the century, threatening to wipe out species such as polar bears.
Researchers say that the Arctic is warming faster than the rest of the globe because darker ground and sea water, once exposed, soak up much more heat than reflective ice and snow.
The WWF has an internet satellite tracking system for Skadi and another bear at: