The death was discovered on Saturday morning, writes Södermanlands Nyheter.
The person was taken to hospital and later pronounced dead. Relatives have been notified. There is no suspicion of a crime, the head of the Swedish Prison and Probation Service, Ann-Katrin Ström, told the newspaper.
The Swedish Prison and Probation Service will now investigate the incident.
The man, who was in his 40s, had been detained on suspicion of, among other things, aggravated weapons offenses and aggravated unlawful threats, Södermanlands Nyheter reports.
The death is the fifth in a Swedish detention center in recent months.
