The incident must have occurred in the Blombacka area at half past eight on Sunday evening.
No one has been arrested at this time.
According to the police, it is a man in his 20s who was seriously injured in a shooting.
- We are on site and checking and interviewing people who should be able to help us in the investigation work, says Mats Eriksson, the police's press officer.
Sunday's shooting is one of several acts of violence that have hit the city in recent weeks.
So far this year, there have been 13 shootings in Södertälje.
The municipality has also received national police reinforcements in the fight against violence.
- We have many extra resources in place, because we know that there is an ongoing conflict here, says Mats Eriksson.
If tonight's shooting is connected to any particular conflict, the police cannot comment on this at this time.
The article is updated.