The Duden (German pronunciation: [ˈduːdn̩] ^ⓘ) is a dictionary of the Standard High German language, first published by Konrad Duden in 1880, and later by Bibliographisches Institut GmbH, which was merged into Cornelsen Verlag in 2022.

The Duden is updated regularly with new editions appearing every four or five years. As of December 2024^[update], it is in its 29th edition. It is printed as twelve volumes, with each volume covering different aspects of the German language such as loanwords, etymology, pronunciation, synonyms, etc.

The first of these volumes, Die deutsche Rechtschreibung (English: The German orthography), has long been the prescriptive source for Standard High German spelling. The Duden has become the most widely used language resource of the Standard High German language, stating the rules regarding grammar, spelling and use of Standard High German language. In Austria, the Österreichisches Wörterbuch takes that role.

In 1872, Konrad Duden, then headmaster of a Gymnasium (secondary school), had his treatise Die deutsche Orthoschrift ("German orthography") published by B.G. Teubner in Leipzig. That book included both a dictionary and spelling rules for school use. Often known as the Schleizer Duden – the author was then the headmaster of a Gymnasium (secondary school) in Schleiz, now in Thuringia — the work significantly influenced a debate about German spelling and became the template for subsequent dictionaries.

Eight years later, having moved to a grammar school in Hersfeld as headmaster, Konrad Duden's main work was published, considerably expanded from the Schleizer Duden. The first edition of this new work, Vollständiges Orthographisches Wörterbuch der deutschen Sprache (Complete Orthographical Dictionary of the German Language), later sometimes referred to by the publisher as Urduden, was published in Leipzig and was the first major complete dictionary of German. This first "Duden" collected 28,000 keywords on 187 pages and subsequently prevailed throughout the German Empire as a standard reference work. From 1892, its spellings also became binding in Switzerland.

In 1902, the Bundesrat confirmed the Duden as the official standard for German spelling; Austria-Hungary and Switzerland soon followed suit. In the ensuing decades, the Duden continued to be the de facto standard for German orthography. After World War II this tradition continued separately in East and West Germany, in Leipzig and Mannheim, respectively.

In West Germany, some publishing houses began to attack the Duden "monopoly" in the 1950s, publishing dictionaries which contained alternative spellings. In reaction, in November 1955, the ministers of culture of the states of Germany confirmed the spellings given by the Duden would continue to be the official standard.

In 1954, the first published Duden appeared in Mannheim, the western counterpart to the traditional Duden printing city of Leipzig. The first East German Duden appeared in Leipzig in 1951, but was largely ignored as illegitimate by West Germany.