Schleswig-Holstein (pronounced [ˌʃleːsvɪç ˈhɔlʃtaɪn] (listen); Danish: Slesvig-Holsten; Low German: Sleswig-Holsteen; North Frisian: Slaswik-Holstiinj) is the northernmost of the 16 states of Germany, comprising most of the historical duchy of Holstein and the southern part of the former Duchy of Schleswig. Its capital city is Kiel; other notable cities are Lübeck and Flensburg.

The region is called Slesvig-Holsten in Danish and pronounced [ˌsle̝ːsvi ˈhʌlˌste̝ˀn]. The Low German name is Sleswig-Holsteen, and the North Frisian name is Slaswik-Holstiinj. Historically, the name can also refer to a larger region, containing both present-day Schleswig-Holstein and the former South Jutland County (Northern Schleswig; now part of the Region of Southern Denmark) in Denmark. It covers an area of 15,763 km² (6,086 sq mi), making it the 5th smallest German federal state by area (including the city-states).

Schleswig was under Danish control during the Viking Age, but in the 12th century it escaped full control and became a duchy. It bordered Holstein, which was a part of the Holy Roman Empire. Beginning in 1460, both Schleswig and Holstein were ruled together by the Danish king acting as duke of both Schleswig and Holstein, with the latter remaining a part of Germany. In the 19th century, Danes and Germans each believed they had a claim to Schleswig-Holstein, the population of which was majority ethnic German. The resulting long-term political and territorial dispute was known as the Schleswig-Holstein Question. In 1848, Denmark tried to formally annex the area. Prussia responded by invading, thus beginning the First Schleswig War, which ended in a victory for Denmark and the signing of the 1852 London Protocol. But the fight broke out again in 1864 (the Second Schleswig War), and this time Prussia and Austria won and the territory was absorbed into Prussia in 1867. More than 50 years later, after the German defeat in World War I, the Allies required that the question of sovereignty over the territory be submitted to plebiscites (the 1920 Schleswig plebiscites), which resulted in the return of some of the territory to Denmark. After World War II, Schleswig-Holstein took in over a million refugees.

Today, Schleswig-Holstein's economy is known for its agriculture, such as its Holstein cows. Its position on the Atlantic Ocean makes it a major trade point and shipbuilding site; it is also the location of the Kiel Canal. Its offshore oil wells and wind farms produce significant amounts of energy. Fishing is a major industry, and the basis of its distinctive unique local cuisine. It is a popular tourist destination for Germans and tourists across the globe.