Warmia (Polish: Warmia [ˈvarmja] ^ⓘ; Latin: Varmia, Warmia; German: Ermland^ⓘ; Warmian: Warńija; Old Prussian: Wārmi) is both a historical and an ethnographic region in northern Poland, forming part of historical Prussia. Its historic capitals were Frombork and Lidzbark Warmiński and the largest city is Olsztyn.

Warmia is currently the core of the Warmian-Masurian Voivodeship (province). The region covers an area of around 4,500 km² (1,700 sq mi) and has approximately 350,000 inhabitants. Important landmarks include the Cathedral Hill in Frombork, the bishops' castles at Olsztyn and Lidzbark, the medieval town of Reszel and the sanctuary in Gietrzwałd, a site of Marian apparitions. Geographically, it is an area of many lakes and lies at the upper Łyna river and on the right bank of Pasłęka, stretching in the northwest to the Vistula Bay. Warmia has a number of architectural monuments ranging from Gothic, Renaissance and Baroque to Classicism, Historicism and Art Nouveau.

Warmia is part of a larger historical region called Prussia, which was inhabited by the Old Prussians and later on was populated mainly by Germans and Poles. Warmia has traditionally strong connections with neighbouring Masuria, but it remained Catholic and belonged directly to Poland between 1454/1466 and 1772, whereas Masuria was a part of Poland as a fief held by the Teutonic Order and Ducal Prussia, which became predominantly Protestant. Warmia has been under the dominion of various states over the course of its history, most notably the Old Prussians, the Teutonic Knights, the Kingdom of Poland and the Kingdom of Prussia. The history of the region is closely connected to that of the Archbishopric of Warmia (formerly Prince-Bishopric of Warmia). The region is associated with the Prussian tribe, the Warmians, who settled in an approximate area. According to folk etymology, Warmia is named after the legendary Prussian chief Warmo, and Ermland derives from his widow Erma.

Warmia is bordered by Powiśle in the west, Masuria in the south and east, and Bartia and Natangia in the north.

Warmia occupies a 100 kilometer long strip of land along the right bank of the Pasłęka River, approximately 20 kilometers wide in the north and increasing to over 70 kilometers wide in the south. The Łyna River drains the southern portion of the region, flowing to the northeast to join with the Pregolya. The terrain is composed of gentle hills and wide plains, and has a humid continental climate, with milder temperatures found at lower elevations in the north near the coast.

With the exception of the far northern and southern ends of the region, the Pasłęka constitutes its western border. That river flows into the Vistula Lagoon just after passing the town of Braniewo. The historically important port town of Frombork lies west of the Pasłęka, near the mouth of the Bauda River [pl]. Further south, the Pasłęka is joined by the tributaries Wałsza [pl] and Drwęca Warmińska, with the headwaters of the river located near the southern end of Warmia.

The source of the Łyna river is found just south of the southern tip of the region, near the eponymous town of Łyna. The river flows through several lakes on the western end of the Masurian Lake District, passing through the cities of Olsztyn and Lidzbark Warmiński as it takes in numerous tributaries on its journey north. This southern portion of Warmia is more heavily forested and historically had many towns with Polish-speaking majorities, while the rest of the region was almost entirely German-speaking.

By the early Middle Ages the Warmians, an Old Prussian tribe, inhabited the area.