Prussia (; German: Preußen, pronounced [ˈpʁɔʏsn̩] (listen), Old Prussian: Prūsa or Prūsija) was a German state located on the southeast coast of the Baltic Sea. It formed the German Empire when it united the German states in 1871. It was de facto dissolved by an emergency decree transferring powers of the Prussian government to German Chancellor Franz von Papen in 1932 and de jure by an Allied decree in 1947. For centuries, the House of Hohenzollern ruled Prussia, expanding its size with the Prussian Army. Prussia, with its capital at Königsberg and then, when it became the Kingdom of Prussia in 1701, Berlin, decisively shaped the history of Germany.

The name Prussia derives from the Old Prussians; in the 13th century, the Teutonic Knights—an organized Catholic medieval military order of German crusaders—conquered the lands inhabited by them. In 1308, the Teutonic Knights conquered the region of Pomerelia with Danzig (modern-day Gdańsk). Their monastic state was mostly Germanised through immigration from central and western Germany, and, in the south, it was Polonised by settlers from Masovia. The imposed Second Peace of Thorn (1466) split Prussia into the western Royal Prussia, becoming a province of Poland, and the eastern part, from 1525 called the Duchy of Prussia, a feudal fief of the Crown of Poland up to 1657. The union of Brandenburg and the Duchy of Prussia in 1618 led to the proclamation of the Kingdom of Prussia in 1701.

Prussia entered the ranks of the great powers shortly after becoming a kingdom. It became increasingly large and powerful in the 18th and 19th centuries. It had a major voice in European affairs under the reign of Frederick the Great (1740–1786). At the Congress of Vienna (1814–15), which redrew the map of Europe following Napoleon's defeat, Prussia acquired rich new territories, including the coal-rich Ruhr. The country then grew rapidly in influence economically and politically, and became the core of the North German Confederation in 1867, and then of the German Empire in 1871. The Kingdom of Prussia was now so large and so dominant in the new Germany that Junkers and other Prussian élites identified more and more as Germans and less as Prussians.

The Kingdom ended in 1918 along with other German monarchies that were terminated by the German Revolution. In the Weimar Republic, the Free State of Prussia lost nearly all of its legal and political importance following the 1932 coup led by Franz von Papen. Subsequently, it was effectively dismantled into Nazi German Gaue in 1935. Nevertheless, some Prussian ministries were kept and Hermann Göring remained in his role as Minister President of Prussia until the end of World War II. Former eastern territories of Germany that made up a significant part of Prussia lost the majority of their German population after 1945 as the Polish People's Republic and the Soviet Union both absorbed these territories and had most of its German inhabitants expelled by 1950. Prussia, deemed "a bearer of militarism and reaction" by the Allies, was officially abolished by an Allied declaration in 1947. The international status of the former eastern territories of the Kingdom of Prussia was disputed until the Treaty on the Final Settlement with Respect to Germany in 1990, but its return to Germany remains a cause among far-right politicians, the Federation of Expellees and various political revanchists and irredentists.

The terms "Prussian" and "Prussianism" have often been used, especially outside Germany, to denote the militarism, military professionalism, aggressiveness, and conservatism of the Junker class of landed aristocrats in the East who dominated first Prussia and then the German Empire.