Kiel (German: [kiːl] (listen)) is the capital and most populous city in the northern German state of Schleswig-Holstein, with a population of 246,243 (2021).

Kiel lies approximately 90 kilometres (56 mi) north of Hamburg. Due to its geographic location in the southeast of the Jutland peninsula on the southwestern shore of the Baltic Sea, Kiel has become one of Germany's major maritime centres, known for a variety of international sailing events, including the annual Kiel Week, which is the biggest sailing event in the world. Kiel is also known for the Kiel Mutiny, when sailors refused to board their vessels in protest against Germany's further participation in World War I, resulting in the abdication of the Kaiser and the formation of the Weimar Republic. The Olympic sailing competitions of the 1936 and the 1972 Summer Olympics were held in the Bay of Kiel.

Kiel has also been one of the traditional homes of the German Navy's Baltic fleet, and continues to be a major high-tech shipbuilding centre. Located in Kiel is the GEOMAR – Helmholtz Centre for Ocean Research Kiel at the University of Kiel. Kiel is an important sea transport hub, thanks to its location on the Kiel Fjord (Kieler Förde) and the busiest artificial waterway in the world, Kiel Canal (Nord-Ostsee-Kanal). A number of passenger ferries to Sweden, Norway, Lithuania and other countries operate from here. Moreover, today Port of Kiel is a popular destination for cruise ships touring the Baltic Sea.

Kiel's recorded history began in the 13th century. Before then, in the eighth century, it was a Danish village. Until 1864 it was administered by Denmark in personal union. In 1866 the city was annexed by Prussia and in 1871 it became part of Germany.

Kiel was one of the founding cities of the original European Green Capital Award in 2006. In 2005 Kiel's GDP per capita was €35,618, which is well above Germany's national average, and 159% of the European Union's average.