The Kingdom of Prussia (German: Königreich Preußen, pronounced [ˈkøːnɪkʁaɪç ˈpʁɔʏsn̩] ^ⓘ) was a German state that existed from 1701 to 1918. It played a significant role in the unification of Germany in 1871 and was a major constituent of the German Empire until its dissolution in 1918. Although it took its name from the region called Prussia, it was based in the Margraviate of Brandenburg. Its capital was Berlin.

The kings of Prussia were from the House of Hohenzollern. The polity of Brandenburg-Prussia, predecessor of the kingdom, became a military power under Frederick William, Elector of Brandenburg, known as "The Great Elector". As a kingdom, Prussia continued its rise to power, especially during the reign of Frederick II "the Great". Frederick the Great was instrumental in starting the Seven Years' War (1756–1763), holding his own against Austria, Russia, France and Sweden and establishing Prussia's dominant role among the German states, as well as establishing the country as a European great power through the victories of the powerful Prussian Army. Prussia made attempts to unify all the German states (excluding the German cantons in Switzerland) under its rule, and whether Austria would be included in such a unified German domain became an ongoing question. After the Napoleonic Wars led to the creation of the German Confederation, the issue of unifying the German states caused the German revolutions of 1848–1849, with representatives from all states attempting to unify under their own constitution. Attempts to create a federation remained unsuccessful and the German Confederation collapsed in 1866 when the Austro-Prussian War ensued between its two most powerful member states.

Prussia was subsequently the driving force behind establishing in 1866 the North German Confederation, transformed in 1871 into the unified German Empire and considered the earliest continual legal predecessor of today's Federal Republic of Germany. The North German Confederation was seen as more of an alliance of military strength in the aftermath of the Austro-Prussian War but many of its laws were later used in the German Empire. The German Empire successfully unified all of the German states aside from Austria and Switzerland under Prussian hegemony due to the defeat of Napoleon III in the Franco-Prussian War of 1870–1871. The war united all the German states against a common enemy, and with the victory came an overwhelming wave of nationalism which changed the opinions of some of those who had been against unification.

With the German Revolution of 1918–1919, the Kingdom of Prussia was transformed into the Free State of Prussia. Prussia as a whole was abolished in 1947.

The Hohenzollerns were made rulers of the Margraviate of Brandenburg in 1518. In 1529, the Hohenzollerns secured the reversion of the Duchy of Pomerania after a series of conflicts, and acquired its eastern part following the Peace of Westphalia.

In 1618, the electors of Brandenburg also inherited the Duchy of Prussia, since 1511 ruled by a younger branch of the House of Hohenzollern. In 1525, Albrecht of Brandenburg, the last grand master of the Teutonic Order, secularized his territory and converted it into a duchy. It was ruled in a personal union with Brandenburg, known as "Brandenburg-Prussia". A full union was not possible, since Brandenburg was still legally part of the Holy Roman Empire and the Duchy of Prussia was a fief of Poland. The Teutonic Order had paid homage to Poland since 1466, and the Hohenzollerns continued to pay homage after secularizing Ducal Prussia.

In the course of the Second Northern War, the treaties of Labiau and Wehlau-Bromberg granted the Hohenzollerns full sovereignty over the Prussian duchy by September 1657.

In return for an alliance against France in the War of the Spanish Succession, the Great Elector's son, Frederick III, was allowed to elevate Prussia to a kingdom in the Crown Treaty of 16 November 1700. Frederick crowned himself "King in Prussia" as Frederick I on 18 January 1701. Legally, no kingdoms could exist in the Holy Roman Empire except for Bohemia and Italy. However, Frederick took the line that since Prussia had never been part of the empire and the Hohenzollerns were fully sovereign over it, he could elevate Prussia to a kingdom. Emperor Leopold I, keen to secure Frederick's support in the impending War of the Spanish Succession, acquiesced.