Olsztyn (UK: /ˈɒlʃtɪn/ OL-shtin, Polish: [ˈɔlʂtɨn] ^ⓘ) is a city on the Łyna River in northern Poland. It is the capital of the Warmian-Masurian Voivodeship, and is a city with county rights. The population of the city was estimated at 169,793 residents in 2021.^[update]

Olsztyn is the largest city in Warmia, and has been the capital of the voivodeship since 1999. In the same year, the University of Warmia and Masuria was founded from the fusion of three other local universities. The city is the seat of the Archbishop of the Roman Catholic Archdiocese of Warmia.

The most important sights of the city include the Old Town with the medieval Castle of Warmian Cathedral Chapter and St. James Co-cathedral, which dates back more than 600 years. The market square is part of the European Route of Brick Gothic and the co-cathedral is regarded as one of the greatest monuments of Gothic architecture in Poland. The city is also known for its association with Nicolaus Copernicus, who lived there and commanded the successful Polish defense of the city during a 1521 Teutonic invasion, with the castle containg an astronomical table made by Copernicus. Today, the castle houses a museum and is a venue for concerts, art exhibitions, film shows and other cultural events, which make Olsztyn a popular tourist destination.

Olsztyn, for a number of years, has been ranked very highly in quality of life, income, employment and safety. It is one of the best places in Poland to live and work. It is also one of the happiest cities in the country.

Teutonic Order 1353–1454
Kingdom of Poland 1454–1455
Teutonic Order 1455–1463
Kingdom of Poland 1463–1569
Polish–Lithuanian Commonwealth 1569–1772
Kingdom of Prussia 1772–1918
German Empire 1871–1918
Weimar Germany 1918–1933
Nazi Germany 1933–1945
People's Republic of Poland 1945–1989
Republic of Poland 1990–present

In 1334, a watchtower was established on the Łyna River. In 1346, the forest was cleared at the location for a new settlement, mentioned in a historical document from 1348. The following year, the Teutonic Knights began the construction of an Ordensburg (castle) as a stronghold against the Baltic Prussians. Allenstein was granted municipal rights by the cathedral chapter of the Bishopric of Warmia in October 1353. The German "Allenstein" referred to the river's Baltic Prussian name Alna, which meant a hind. Local Poles, having arrived along with German settlers, called it Holstin and Olsztyn, which are Polonizations of the German name. The castle was completed in 1397. The town was captured by the Kingdom of Poland during the Polish-Lithuanian-Teutonic War in 1410, and again in 1414 during the Hunger War, but it was returned to the monastic state of the Teutonic Knights after hostilities ended.

The city joined the Prussian Confederation in 1440, and rebelled against the Teutonic Knights in 1454 upon the outbreak of the Thirteen Years' War to join the Kingdom of Poland under King Casimir IV Jagiellon. In 1454, upon the request of the Confederation, King Casimir IV signed the act of incorporation of the region to Poland, and the townspeople took the castle and recognized the Polish king as the rightful ruler. Although the Teutonic Knights recaptured the city the following year, it was retaken by Polish troops in 1463. The Second Peace of Thorn in 1466 confirmed Allenstein as part of the Kingdom of Poland. Administratively it was located in the Prince-Bishopric of Warmia within the provinces of Royal Prussia and Greater Poland.

From 1516 to 1521, Nicolaus Copernicus lived in the town castle as an administrator and then in Mehlsack (Melzak, now Pieniężno). Copernicus was in charge of the Polish defences in the Siege of Allenstein during the Polish-Teutonic War of 1519–21. He also started and managed the repopulation of the region, inviting a new wave of Polish settlers from Mazovia. The town along with Warmia then entered what is considered the region's golden age, when crafts and trade developed, thanks also to the city's location on the Warsaw-Königsberg trade route. During this period, the city was still visited several times by Copernicus, as well as leading figures of the Polish Renaissance, writers, royal secretaries and diplomats: Johannes Dantiscus, called the "father of Polish diplomacy", and Marcin Kromer, who was also a historian and music theorist. St. James' Pro-Cathedral, one of the most distinctive landmarks of the cityscape, was completed at that time.