Słupsk (Polish: [swupsk] ^ⓘ; Kashubian: Stôłpsk [stɞwpsk]; German: Stolp [ʃtɔlp]) is a city with powiat rights located on the Słupia River in the Pomeranian Voivodeship in northern Poland, in the historical region of Pomerania or more specifically in its part known in contemporary Poland as Central Pomerania (Pomorze Środkowe) within the wider West Pomerania (Pomorze Zachodnie). According to Statistics Poland, it has a population of 88,835 inhabitants while occupying 43.15 square kilometres (16.66 sq mi), thus being one of the most densely populated cities in the country as of December 2021. In addition, the city is the administrative seat of Słupsk County and the rural Gmina Słupsk, despite belonging to neither.

Słupsk had its origins as a Pomeranian settlement in the early Middle Ages. In 1265, it was given city rights. By the 14th century, the city had become a centre of local administration and trade and a Hanseatic League associate. Between 1368 and 1478, it was a residence of the Dukes of Słupsk, until 1474 vassals of the Kingdom of Poland. According to the peace treaty of 1648, Słupsk became part of Brandenburg-Prussia. In 1815, it was incorporated into the newly formed Prussian Province of Pomerania. After World War II, the city again became part of Poland, and from 1975 to 1998 it was the capital of Słupsk Voivodeship. It is a railway junction located on the main railroad between the Tricity and Szczecin.

Słupsk contains the oldest preserved industrial structure in Poland and a Renaissance Ducal Castle, which houses the Museum of Middle Pomerania with the largest collection of paintings by popular early-20th-century artist Stanisław Ignacy Witkiewicz. It is also home to the Pomeranian University in Słupsk and notable basketball club Czarni Słupsk.

Slavic names in Pomeranian — Stolpsk, Stôłpsk, Słëpsk, Słëpskò, Stôłp — and Polish — Słupsk — may be etymologically related to the words słup 'pole' and stołp 'keep'. There are two hypotheses about the origin of those names: that it refers to a specific way of constructing buildings on boggy ground with additional pile support, which is still in use, or that it is connected with a tower or other defensive structure on the banks of the Słupia River.

Later, under German administration, the town was named Stolp, to which the suffix in Pommern was attached in order to avoid confusion with other places similarly named. The Germanised name comes from one of five Slavic Pomeranian names of this settlement. The city was occasionally called Stolpe, referring to the Słupia River, whose German name is Stolpe. Stolpe is also the Latin exonym for this place.

Słupsk developed from a few medieval settlements located on the banks of the Słupia River, at the unique ford along the trade route connecting the territories of modern Pomeranian and West Pomeranian Voivodeships. This factor led to the construction of a grod, a West Slavic or Lechitic fortified settlement, on an islet in the middle of the river. Surrounded by swamps and mires, the fortress had perfect defence conditions. Archaeological research has shown that the grod was situated on an artificial hill and had a natural moat formed by the branches of the Słupia, and was protected by a palisade. Records confirm that the area of Słupsk was part of the Polish realm during the reign of Mieszko I and in the 11th century.

According to several sources, the first historic reference to Słupsk comes from the year 1015 when the king of Poland Boleslaus I the Brave took over the town, incorporating it into the Polish state. In the 12th century, the town became one of the most important castellanies in Pomerania alongside Gdańsk and Świecie. However, several historians stated that the first mention was in two documents dating to 1227, signed by the Pomeranian dukes Wartislaw III and Barnim I and their mothers, confirming the establishment of an abbey in 1224 and donating estates, among them a village "in Stolp minore" or "in parvo Ztolp", respectively, to that abbey. Another document dated to 1180, which mentions a "castellania Slupensis" and would thus be the oldest surviving record, has been identified as a late 13th-century or 14th-century duplicate.

The Griffin dukes lost the area to the Samborides during the following years, and the next surviving documents mentioning the area concern donations made by Samboride Świętopełk II, dating to 1236 (two documents) and 1240. In the earlier of the two 1236 documents, a Johann "castellanus de Slupcz" is mentioned as a witness, Schmidt considers this to be the earliest mention of the gard, since a castellany required the existence of a gard. The first surviving record explicitly mentioning the gard is from 1269: it notes a "Christianus, castellanus in castro Stolpis, et Hermannus, capellanus in civitate ante castrum predictum", thus confirming the existence of a fortress ("castrum") with a suburbium ("civitas"). Schmidt further says that the office of a capellanus required a church, which he identifies as Saint Peter's. This church is mentioned by name for the first time in a 1281 document of Samboride Mestwin II, which also mentions Saint Nicolai church and a Saint Mary's chapel in the fortress. The oldest mention of Saint Nicolai church dates to 1276.