Royal Prussia (Polish: Prusy Królewskie; German: Königlich-Preußen or Preußen Königlichen Anteils, Kashubian: Królewsczé Prësë) or Polish Prussia (Polish: Prusy Polskie; German: Polnisch-Preußen) was a province of the Crown of the Kingdom of Poland, which was established following the Second Peace of Thorn (1466) from territory in Pomerelia and western Prussia which had been part of the State of the Teutonic Order. Royal Prussia retained its autonomy, governing itself and maintaining its own laws, customs, and rights.

In 1569, Royal Prussia was fully integrated into the Crown of the Kingdom of Poland and its autonomy was largely abolished. As a result, the Royal Prussian parliament was incorporated into the Sejm of the Polish–Lithuanian Commonwealth. In 1772 and 1793, after the first and second partition of Poland, the former territory of Royal Prussia was annexed by the Kingdom of Prussia and subsequently re-organized into the province of West Prussia.

The area consisted of the following territories:

In contrast, southern remainder of Pomesania and Pogesania, including Marienwerder (Kwidzyn), Deutsch Eylau (Iława), Riesenburg (Prabuty), Rosenberg (Susz), Bischofswerder (Biskupiec), Saalfeld (Zełwałd, later Zalewo), Freystadt (Kisielice), Mohrungen (Morąg), Preußisch Holland (Pasłęk) and Liebstadt (Libsztat, later Miłakowo), formed Upper Prussia (German: Oberland, Polish: Prusy Górne) constituting the westernmost part of the Prussian territory left to the Teutonic Knights, known as Teutonic or Monastic Prussia with Königsberg as its capital, later secularized in 1525 to become Ducal Prussia ruled by the Protestant dukes of the Hohenzollern dynasty. From 1618, this area was ruled in personal union with the Electorate of Brandenburg as Brandenburg-Prussia. In 1657, the Treaty of Wehlau granted full independence to the Duchy of Prussia, allowing its later elevation to Kingdom in Prussia.

Originally Polish, the Pomerelian part of the region was gradually emancipating during the fragmentation of Poland upon the death of Bolesław III Wrymouth in 1138. Duchy of Pomerelia became fully independent in 1227, when a troop of Duke Świętopełk I of Pomerania attacked the assembly of the Polish princes and murdered the High Duke Leszek the White. The duchy did not maintain its unity and was divided among the male members of the ducal family. All of Pomerelia came under the rule of Mściwój II in 1273. In 1282, he paid homage to Przemysł II, Duke of Greater Poland, soon to be King of Poland.

During the rule of Władysław I the Elbow-high, the Margraviate of Brandenburg challenged his rule over the territory in 1308, leading Władysław to request assistance from the Teutonic Knights, who ousted the Brandenburgers, but then seized Pomerelia for themselves and incorporated it into the Teutonic Order state in 1309. The papal court of 1320 ordered that Pomerelia be returned to Poland. The arbitration of the kings of Bohemia and Hungary in 1335 recognized the province as a perpetual alms granted by the King of Poland to the Teutonic Order. Another papal court in 1339 awarded Poland the right to all the lands seized by the Teutonic Order and ordered their restitution.

At the beginning of the 15th century, the lands held by the Teutonic Knights were inhabited as a whole by a mixed population; it is estimated that there were about 200 000 Germans in the state altogether, followed by 140 000 native Prussians located in the Prussia proper (east of Vistula), as well about 140 000 Poles in Pomerelia and Masuria. About 110,000 people lived in the 93 towns of the monastic state, that is, nearly one quarter of the population. Proper Prussia was inhabited by 270,000 people, Pomerelia by 130,000, and the Chełmno Land by 80,000. In Pomerelia, Poles constituted two-thirds of the population, and in the Chełmno Land one-half. In the 15th century, a process began of migration from Poland and Lithuania, as well as the gradual assimilation of the local ethnic Prussian population with other groups.

The burden of taxation and the arbitrary way of governing caused resistance among the people of Prussia. The burghers of the great Prussian cities began to organize themselves. The first organized body was the Lizard League, founded by the Chelmno Land nobility in 1397. After being defeated at the Battle of Grunwald in 1410, the Teutonic Knights' prestige declined; most towns and castles, as well as three Prussian bishops, swore loyalty to the Polish king. Although the Order soon regained control over most of its territory, by the 1411 Peace of Thorn it was forced to pay large compensation of 100,000 kop groszy for the return of prisoners, which became a financial burden on the citizenry. Facing the opposition, the Komtur of Danzig ordered the execution without a trial of the city's mayor, Konrad Letzkau, along with two councillors and five Chełmno nobles.