Kassel, spelled Cassel until 1926, is a city on the Fulda River in northern Hesse, Germany. It is the administrative seat of the Regierungsbezirk Kassel and the district of the same name and had 200,507 inhabitants in December 2015. The former capital of the state of Hesse-Kassel has many palaces and parks, including the Bergpark Wilhelmshöhe, which is a UNESCO World Heritage Site. Kassel is also known for the documenta exhibitions of contemporary art. Kassel has a public university with 25,000 students (2018) and a multicultural population (39% of the citizens in 2017 had a migration background).

Kassel is the largest city in the north of the federated state of Hesse in the south-western part of Germany, about 70 kilometers northwest of the geographic center of Germany.

It is located on both sides of the river Fulda. Kassel's deepest point is in the north-eastern Fulda valley at 132.9 m above sea level.

The urban area of Kassel is divided into 23 local districts, each of which has a local council with a local mayor as chairman. The local councils are elected every five years by the population of the local districts. The local advisory board can be heard on all important issues affecting the local district. However, the final decision on a measure rests with the Kassel city council.

In 1558, the first German observatory was built in Kassel, and a later version from 1714 survives as the Bellevue Palace. The Ottoneum, the first permanent German theatre building, was built in 1604. The old building is today the Natural History Museum, and the now-called Staatstheater Kassel is located in a nearby building that was constructed in the 1950s. Since 1927, Kassel has been home to Barenreiter, one of the world's most important music publishers.

Since 1955 the Documenta, an international exhibition of modern and contemporary art, has been held regularly in Kassel. The Documenta now takes place every five years. The most recent exhibition, documenta 14 is being held from June to September, 2017. As a result of the Documenta 6 (1977), Kassel became the first town in the world to be illuminated by laser beams at night (Laserscape, by artist Horst H. Baumann). This laser installation is nowadays still visible at weekends. Other Documenta remnants (mainly sculptures) can be found in many places in Kassel; among those are the "7000 Oaks", a work of land art by the German artist Joseph Beuys. All over the city are art pieces from former Documenta exhibitions. Currently the city plans to construct a Documenta institute connected to the university.