Kwidzyn (Polish pronunciation: [ˈkfʲid͡zɨn] ^ⓘ; German: Marienwerder [maˌʁiːənˈvɛʁdɐ] ^ⓘ) is a town in northern Poland on the Liwa River. With a population of 36,731, it is the capital of Kwidzyn County in the Pomeranian Voivodeship.

Kwidzyn is located on the Liwa River, some 5 kilometres (3.1 miles) east of the Vistula river, approximately 70 km (43 mi) south of Gdańsk and 145 km (90 mi) southwest of Kaliningrad. It is part of the region of Powiśle.

The Pomesanian settlement called Kwedis existed in the 11th century. In 1232, the Teutonic Knights built the castle and established the town of Marienwerder (now Kwidzyn) the following year. In 1243, the Bishopric of Pomesania received both the town and castle from the Teutonic Order as fiefs, and the settlement became the seat of the Bishops of Pomesania within Prussia. The town was populated by artisans and traders, originating from towns in the northern parts of the Holy Roman Empire. A Teutonic knight, Werner von Orseln, was murdered in Marienburg (Malbork) in 1330. He was among the first to be buried in the newly erected cathedral of the town.

St. Dorothea of Montau lived in Marienwerder from 1391 until her death in 1394; future pilgrims visiting her shrine would contribute to the flourishing economy.

The Prussian Confederation, which opposed Teutonic rule, was founded in the town on March 14, 1440. The town itself joined the organization on 17 April 1440. Upon the request of the organization in 1454 Polish King Casimir IV Jagiellon incorporated the region and town to the Kingdom of Poland, and the Thirteen Years' War broke out. In 1466, after the defeat of the Teutonic Knights in the war, the town became part of Poland as a fief held by the Teutonic Knights. In 1525, the Teutonic state was transformed into a secular and Lutheran duchy under the last Grand Master of the Teutonic Order Albert, a political foundation only possible with the consent of the Polish King Sigismund I the Old. The town was visited by Polish Kings Sigismund II Augustus in 1552 and Stephen Báthory in 1576. In 1618 the ducal rights were inherited by the Brandenburg branch of the House of Hohenzollern, remaining under Polish suzerainty. In 1657 the Brandenburg dukes severed ties with the Polish crown and in 1701 elevated their realm to the sovereign Kingdom of Prussia. During the War of the Polish Succession, Polish King Stanisław Leszczyński stayed in the town in July 1734.

In 1765 Prussia established a customs chamber for Polish products floated down the Vistula to Polish Baltic ports. The town of Marienwerder meanwhile had become the capital of the District of Marienwerder. In 1772, the Marienwerder district was integrated into the newly established Prussian Province of West Prussia, which consisted mostly of territories annexed in the First Partition of Poland. In November 1831, several Polish cavalry units of the November Uprising stopped in the town on the way to their internment places.

By the enlargement of its administrative functions, the population of the town started to grow and in 1885, it numbered 8,079. This population was composed mostly of Lutheran inhabitants, many of whom were engaged in trades connected with the manufacturing of sugar, vinegar and brewing as well as dairy farming, fruit growing and the industrial construction of machines. In 1910, Marienwerder had a population of 12,983 of which 12,408 (95.6%) were German-speaking and 346 (2.7%) were Polish-speaking.

As a result of the Treaty of Versailles after World War I, the district of Marienwerder was divided. The parts west of the Vistula were incorporated into the Polish Second Republic, which had just regained its independence. The parts east of the Vistula, to which the town of Marienwerder belonged, was to take part in the East Prussian plebiscite, which was organized under the control of the League of Nations. The Inter-Allied Commission with nearly 2,000 troops often favored the Germans, and its services towards Poles were often delayed and limited, while the administration remained under German control. The town was home to the Polish Warmian Plebiscite Committee and the Committee for Polish Affairs, which, however, had to operate partly secretly. On May 16, 1920, the largest Polish plebiscite demonstration in Powiśle took place in the town, and Poles had to organize defenses against attacks by German militias. According to Polish sources there was German electoral fraud. In Marienwerder 7,811 votes were given to remain in East Prussia, and therefore Germany, and 362 for Poland. Afterwards, anti-Polish terror intensified.