The Second Schleswig War (Danish: Krigen i 1864; German: Deutsch-Dänischer Krieg) also sometimes known as the Dano-Prussian War or Prusso-Danish War was the second military conflict over the Schleswig-Holstein Question of the nineteenth century. The war began on 1 February 1864, when Prussian and Austrian forces crossed the border into Schleswig. Denmark fought the Kingdom of Prussia and the Austrian Empire. Like the First Schleswig War (1848–1852), it was fought for control of the duchies of Schleswig, Holstein and Lauenburg, due to the succession disputes concerning them when the Danish king died without an heir acceptable to the German Confederation. The war started after the passing of the November Constitution of 1863, which integrated the Duchy of Schleswig into the Danish kingdom in violation of the London Protocol.

The war ended on 30 October 1864, with the Treaty of Vienna and Denmark's cession of the Duchies of Schleswig (except for the island of Ærø, which remained Danish), Holstein and Saxe-Lauenburg to Prussia and Austria.