The man had been admitted to a psychiatric clinic after running around on a roof armed with a knife.
He was taken to hospital where he died on Friday, reports the Helsingborgs Dagblad newspaper.
According to the police report, the door to his room at the psychiatric ward was locked.
“You can’t leave a mentally ill person alone, it’s as stupid as leaving a person who suffered a heart attack,” said one of the man’s relatives to the newspaper.
The hospital has launched an internal investigation into the suicide and plans to report the matter to the National Board of Health and Welfare (Socialstyrelsen) in accordance with Sweden’s Lex Maria rules.