Two more people have been hospitalized and are reportedly in severe condition, reported the Östergötland police's communication centre (LKC).
Gothenburg's sea rescue centre received a call at 9pm, reporting that the boat had begun taking in water after running aground. Onboard were three people, a man, a woman and their daughter, as well as the family's dog.
Both rescue boats and helicopters were dispatched, and were on the scene of the accident by 10pm, according to newspaper Dagens Nyheter.
However, it took some time to find the boat, which was then already partially under water.
Two people, the man and one of the women, were found relatively quickly nearby the boat, both suffering from hypothermia, and immediately rushed to hospital.
The second woman wasn't found until 11.30pm, and was unconscious when raised out of the water, reported the paper.
She later died at Nyköpings hospital, where one more person is being cared for. The third person was taken to Vrinnevi hospital.
The police have yet to identify the victims.
The dog drowned.