Police received the first reports of the incident in Pildammsparken at 12.30pm on Sunday, after which the man was taken to hospital and the area cordoned off for forensic investigation.
“Paramedics on the scene immediately carried out cardiopulmonary resuscitation, but we don't yet know how the person is doing,” Patric Fors, a police press spokesman, told the local Kvällsposten newspaper.
“The police do not suspect that a crime has been committed,” his colleague Calle Persson told the TT newswire.
Pildammsparken was created for the Baltic Exhibition in 1914 and was designed by Sweden's then Crown Princess Margareta together with the famed architect Ferdinand Boberg. It combines a beech wood with formal gardens and ornamental ponds.