Kaliningrad Oblast (Russian: Калининградская область, romanized: Kaliningradskaya oblastʹ) is the westernmost federal subject of Russia. It is a semi-exclave on the Baltic Sea within the historical Baltic region of Prussia, bordered by Poland to the south Lithuania to the north and east, and the Baltic Sea to the west. The largest city and administrative centre is the city of Kaliningrad. The port city of Baltiysk is Russia's only port on the Baltic Sea that remains ice-free in winter. Kaliningrad Oblast had a population of roughly one million in the 2021 Russian census. It has an area of 15,125 square kilometres (5,840 sq mi).

Various peoples, including Lithuanians, Germans, and Poles, lived on the land which is now Kaliningrad. The territory was formerly the northern part of East Prussia. With the defeat of Nazi Germany in World War II, the territory was annexed to the Russian SFSR by the Soviet Union. Following the post-war migration and flight and expulsion of Germans, the territory was populated with Soviet citizens, mostly Russians.

The territory of what is now the Kaliningrad Oblast used to be inhabited by the Old Prussians and other Western Balts, prior to the Teutonic conquest in the early Late Middle Ages. Afterwards, it was settled by Germans (especially the western part), Lithuanians (especially Lithuania Minor) and Poles (especially Königsberg, Polish: Królewiec, and the current southern border strip). The Old Prussians became extinct due to Germanisation in the first half of the 18th century. The Lithuanian-inhabited areas of the Teutonic State were known as Lithuania Minor, which encompassed all of modern Kaliningrad Oblast until the 18th century.

In the 13th century, the Teutonic Order conquered the region and established the State of the Teutonic Order, a theocracy. In 1255, on the foundations of a destroyed Sambian settlement known as Tvanksta, the Teutonic Order founded the city of Königsberg (modern Kaliningrad), naming it in honour of Ottokar II of Bohemia.

The Northern Crusades, including the Lithuanian Crusade, were partly motivated by colonization. The German colonist peasants, craftsmen, and merchants were predominantly concentrated in the southern part of the Teutonic State and did not move into Nadruvia and Skalvia due to the Lithuanian military threat.

In 1454, following a request by the anti-Teutonic Prussian Confederation, the territory was incorporated to the Kingdom of Poland by King Casimir IV Jagiellon, an event that sparked the Thirteen Years' War (1454–1466). After Poland's victory in the war with the Second Peace of Thorn, the State of the Teutonic Order became a vassal of Poland, also considered an integral part of "one and indivisible" Kingdom of Poland. During this war, the capital of the Teutonic state was moved from Marienburg (now Malbork) to Königsberg in 1457. When the rulers of Prussia were vassals of the King of Poland from 1466 to 1660, there were few German colonists.

After the Teutonic Order lost the war of 1519–1521 with Poland, the Teutonic Order remained a vassal of the Kingdom of Poland. In 1525, Grand Master Albert of Brandenburg secularized the Teutonic Order's Prussian branch and established himself as ruler of the Duchy of Prussia, the first Protestant state in Europe. Königsberg was the residence of the Duke of Prussia from 1525 until 1701, and was the Duchy of Prussia's capital until 1660, when the capital moved to Berlin.

Polish and Lithuanian culture blossomed in Königsberg, with the city being the place of publication of the first Polish- and Lithuanian-language cathechisms (by Jan Seklucjan and Martynas Mažvydas), the first Polish translation of the New Testament, Grammatica Litvanica, the first Lithuanian grammar book, and the Albertina University being the second oldest university of the Polish–Lithuanian Commonwealth, after receiving a royal privilege from King Sigismund II Augustus in 1560. Polish printing continued for centuries with the last Polish publication in 1931.