Sępólno Krajeńskie pronounced [sɛmˈpulnɔ kraˈjɛɲskʲɛ] (German: Zempelburg) is a town in northern Poland, in the Kuyavian-Pomeranian Voivodeship.

[2] The city is located in the historical Krajna forest on a high bank of the Sępólna River.

The town formed part of the Kalisz Voivodeship of the Greater Poland Province of the Polish Crown from 1314 to 1793.

The Catholic church, mentioned as early as 1360, suggests that it was located in the Sępólna River valley.

According to legend, the castle manor was lost when the river and nearby Dziechowo Lake flooded.

[4] Sępólno was part of the short-lived Polish Duchy of Warsaw in 1807–1815 during the Napoleonic Wars, and afterwards it was re-annexed by Prussia.

After the First World War, Zempelburg had to be ceded to Poland without a referendum due to the provisions of the Versailles Treaty in 1920 to establish the Polish Corridor.

In 1920, the eastern part of the former Flatow district with the towns of Kamień Krajeński, Więcbork and Sępólno was reintegrated with the restored Polish Republic after the Treaty of Versailles.

Sępólno was invaded by Nazi Germany on September 1, 1939,[4] the first day of World War II, and was later annexed and made the seat of Zempelburg district within the Reichsgau Danzig-West Prussia.

During the German occupation, Poles were subject to persecutions, mass arrests, Germanisation, expulsions and massacres.

[4] Mass executions of Poles in Sępólno were carried out in various places, for example on the railway tracks connecting Sępólno and Kamień Krajeński (in October 1939), at the primary school and at the shooting range (in November 1939),[7] local Poles were also murdered in Radzim, Karolewo, Rudzki Most.