At 02:00 on Saturday night, the police, rescue service and ambulance were alerted to Riddarhyttan in Skinnskatteberg.
Two people had been out on a bathing jetty, according to the police in Västmanland.
One man jumped into the water but did not get up again.
Then the other person at the scene called SOS Alarm.
After a period of searching in Lake Lien, the man in his 50s was found dead.
The man's relatives have been notified.