"I've never seen this before. It is exceptional," said Lars Andersson, an oceanographer at the national weather service SMHI.
In the area of Skagerrak, the strait running between the southeast coast of Norway and the southwest coast of Sweden, the surface water was 13.5 degrees Celsius last week.
That's more than 4 degrees warmer than normal. The water was just as warm at 100 meters below the surface, TT reports.
Other areas experiencing higher temperatures were Sweden's southern coastline along the Baltic Sea and Kattegat, the waters between Sweden and Northern Denmark.
The unusually warm water could mean that ice will form later this year and that low-pressure weather systems could be more powerful.