At 23.28, several police patrols and an ambulance were alerted to Brandkärr in Nyköping after a suspected shooting.
When the police arrived, an injured man was found, who was taken by ambulance to hospital.
The man's condition is critical.
Police have cordoned off the area and technicians will investigate the site.
No one has been arrested yet.
The police urge any witnesses to contact 114 14.