It was shortly after 10 pm that the police and ambulance were alerted to Vrena, between Nyköping and Katrineholm.
Witnesses in a car behind them had then seen a man being hit and a car disappearing from the scene.
- We take this very seriously.
It could have been much worse, says police spokesman Björn Öberg.
The man who was hit, who is in his 50s, ended up in the ditch after the accident.
He was taken to hospital but the injuries should not be life-threatening.
At the accident site, certain tracks from the car that were seen driving on the pedestrian were secured.
A report of infection from a traffic accident site and causing bodily injury has been made.