The incident occurred near Kornhamnstorg square in the popular tourist district of the Swedish capital, according to police.
The stabbing prompted a massive police response and two people who had initially fled the scene were later arrested.
The victim was outside at the time of the stabbing and was conscious when an ambulance took him to hospital. The extent of the man's injuries remains unknown.
They are now suspected of attempted murder, with an alternative charge of manslaughter.
"Now the two will be interrogated. We still don't know anything about the motive or any other related information," police spokesman Björn Dahlberg told the TT news agency.
The attack was seen by several witnesses, according to police, who have yet to establish the identity of the victim.
Preliminary reports from the police indicated that the stabbing victim was a woman, but Dahlberg later explained the information was the result of a misunderstanding.