The man was found on the Södermalm island in the southern central part of the city at around 5am by a police patrol.
He was taken to hospital, and while the cause of his injuries is currently unclear, police have launched a preliminary investigation into attempted murder.
“There's no more we can say at the moment. We found out about it at 5am and are working on it just now,” Stockholm police spokesperson Eva Nilsson told news agency TT.
The site where the man was found lying injured is cordoned off for a forensic examination.