town, in the län (county) of Stockholm, east-central Sweden. It lies between a bay of Lake Mälar and the Baltic Sea, southwest of Stockholm. The town, formerly called simply Tälje, was founded in the 10th century and was damaged by fire in 1390, 1650, and 1719. In and around the town are St. Ragnhild's Church (dating from about 1200), Gripsholm Castle, and an open-air museum.

In the 19th century Södertälje was a spa and bathing resort, but it is now a manufacturing centre and a rail and road junction on the routes from Stockholm to Malmö, Göteborg, and Oslo. Lake Mälar and the Baltic are connected at Södertälje by the 3-mile- (5-km-) long Södertälje Canal. Pop. (2005 est.) mun., 80,553.