Sigismund III Vasa became king of both Sweden and the Polish–Lithuanian Commonwealth in a personal union upon his election to the Swedish throne in 1592 amidst much controversy and religious strife.

The renewed controversy came to a head in 1594, resulting in the Riksdag setting aside his authority, and the whole history of Sigismund with his claim on the Swedish throne became a succession of trigger events leading to the subsequent seven decades of dynastic warfare known as the Polish–Swedish wars.

The same Riksdag approved the execution of eight of Charles IX's opponents in the Swedish nobility as allies of King Sigismund.

In the early middle-ages it was the most important port in Sweden at the confluence of the Storån and Lillån rivers trading mainly with Lubeck, principally in salt, textiles, butter and beer.

Today Söderköping is visited by foreign tourists taking boat tours of the Göta Kanal and is a popular destination for residents of the area.