Sępólno Krajeńskie [sɛmˈpulnɔ kraˈjɛɲskʲɛ] (German: Zempelburg) is a town in Poland, in the Kuyavian-Pomeranian Voivodeship, about 63 km northwest of Bydgoszcz. It is the capital of Sępólno County (powiat sępoleński) and has a population of 9,174 (2004).
The town was founded by the Teutonic Knights in 1309 as Zempelburg and received Magdeburg rights in 1360. It was located in the valley of the Zempolna River (Sępólna), which flooded the old castle because of its proximity to a nearby lake.
In 1466 the town passed to Poland as part of the province of Royal Prussia. It was annexed by the Kingdom of Prussia during the First Partition of Poland in 1772. Zempelburg became part of the Duchy of Warsaw from 1807-1815 during the Napoleonic Wars, after which it was restored to Prussia. In 1871 the town became part of the Prussian-led German Empire. Zempelburg was a center for the textile and shoemaking industries.
Zempelburg belonged to Landkreis Flatow until 1920, when it was transferred to the Second Polish Republic after the Treaty of Versailles. The town, known as Sępólno in Polish, became the seat of Sępólno County. Sępólno was annexed by Nazi Germany in 1939 and made the seat of Landkreis Zempelburg. The town was liberated in 1945 and restored to Poland.
Landmarks of Sępólno Krajeńskie include a Protestant church in the market built in 1857.