The most direct shipping route from Europe to Asia is fully clear of ice for the first time since records began, the European Space Agency (Esa) says.
Historically, the Northwest Passage linking the Atlantic and Pacific Oceans has been ice-bound through the year.
But the agency says ice cover has been steadily shrinking, and this summer's reduction has made the route navigable.
The findings, based on satellite images, raised concerns about the speed of global warming.
The Northwest Passage is one of the most fabled sea routes in the world - a short cut from Europe to Asia through the Canadian Arctic.
Recent years have seen a marked shrinkage in its ice cover, but this year it was extreme, Esa says.
It says this made the passage "fully navigable" for the first time since monitoring began in 1978.
"We have seen the ice-covered area drop to just around 3m sq km (1.2m sq miles)," Leif Toudal Pedersen of the Danish National Space Centre said.
He said it was "about 1m sq km (386,000 sq miles) less than the previous minima of 2005 and 2006".
"There has been a reduction of the ice cover over the last 10 years of about 100, 000 sq km (38,600 sq miles) per year on average, so a drop of 1m sq km (386,000 sq miles) in just one year is extreme," Mr Pedersen said.
The Northeast Passage through the Russian Arctic has also seen its ice cover shrink and it currently "remains only partially blocked," Esa says.
'Battle for Arctic'
Scientists have linked the changes to global warming which may be progressing faster than expected.
The opening of the sea routes is already leading to international disputes.
Canada says it has full rights over those parts of the Northwest Passage that pass through its territory and that it can bar transit there.
But this has been disputed by the US and the European Union.
They argue that the new route should be an international strait that any vessel can use.