Adolf Köster (8 March 1883, Verden an der Aller — 18 February 1930) was a German politician of the Social Democratic Party of Germany (SPD) and diplomat. He served as Foreign Minister of Germany (1920) and Interior Minister of Germany (1921–1922).
Born in Verden an der Aller, he grew up in Kappeln and became a member of the SPD as a youth. He taught at the Technical University of Munich until 1912, and during the First World War, he was a war correspondent for the SPD daily Vorwärts.
Following the November Revolution in 1918, he joined the Prussian State Chancellory, and was appointed as State Secretary in the German Chancellory in May 1919. He served as Foreign Minister of Germany from 10 April to 8 June 1920 in the first cabinet of Hermann Müller that consisted of the SPD, the Catholic Centre Party and the liberal German Democratic Party (DDP).
In the second cabinet of Joseph Wirth (Centre, SPD, DDP), he served as Minister of the Interior of Germany from 26 October 1921 to 14 November 1922.
In 1928, he was appointed as Envoy to Belgrade. He died in Belgrade in 1930.
- Fort mit der Dolchstoßlegende! Warum wir 1918 nicht weiterkämpfen konnten. Berlin: Verlag für Politik und Wissenschaft, 1922.
- Mit den Bulgaren. Kiegsberichte aus Serbien und Mazedonien. [Verlag] Albert Langen, München, 1916. 148 S.
- Wandernde Erde. Kriegsberichte aus dem Westen. Verlag Albert Langen, München, 1917. 133 S.