Karelia (/kəˈriːliə, kəˈriːljə/; Karelian and Finnish: Karjala [ˈkɑrjɑlɑ]; Russian: Каре́лия, romanized: Kareliya [kɐˈrʲelʲɪjə], historically Коре́ла, Korela [kɐˈrʲelʲə]; Swedish: Karelen [kɑˈreːlen]) is an area in Northern Europe of historical significance for Russia (including the Soviet era), Finland, and Sweden. It is divided between northwestern Russia (the federal subjects of the Republic of Karelia and Leningrad Oblast) and Finland (the regions of South Karelia, North Karelia, and the eastern portion of Kymenlaakso).

Various regions may be called Karelia. Finnish Karelia is a historical province of Finland and is now divided between Finland and Russia, often called just Karjala in Finnish. The eastern part of this chiefly Lutheran area was ceded to Russia after the Winter War of 1939–40.

The Republic of Karelia is a Russian federal subject, including East Karelia, with a chiefly Russian Orthodox population.

Within present-day Finland, Karjala refers to the regions of South and North Karelia, although parts of historical Karelia also lie within the region of Kymenlaakso (east of the River Kymi), North Savo (Kaavi, Rautavaara and Säyneinen), and South Savo (Mäntyharju).

Karelia stretches from the White Sea coast to the Gulf of Finland. It contains the two largest lakes in Europe, Lake Ladoga and Lake Onega. The Karelian Isthmus is located between the Gulf of Finland and Lake Ladoga. The highest point of Karelia, the 576-metre-high (1,890 ft) Nuorunen, is located on the Russian side of the Maanselkä hill region.

The border between Karelia and Ingria, the land of the closely related Ingrian people, had originally been the Neva River itself but later on it was moved northward into the Karelian isthmus to follow the Sestra River (Russian: Сестра), today in the Saint Petersburg metropolitan area, but in 1812–1940 the Russo-Finnish border.

On the other side of Lake Ladoga, the River Svir is usually thought of as the traditional southern border of Karelian territory while Lake Onega and the White Sea mark the Eastern border. The River Kymi marks the historic western border of Karelian territory as it served as the boundary between the Häme Finns and the Karelians during the Middle Ages. The River Kymi is also said to have formed a boundary between the eastern and western cultural spheres by the beginning of the Bronze Age at the latest. In the North lived the nomadic Samis, but there were no natural borders except for large wooded areas (taiga) and the tundra.

In historical texts, Karelia is sometimes divided into East Karelia and West Karelia, which are also called Russian Karelia and Finnish Karelia respectively. The area to the north of Lake Ladoga which belonged to Finland before World War II is called Ladoga Karelia, and the parishes on the old pre-war border are sometimes called Border Karelia. White Karelia (sometimes the Finnish or Karelian term "Viena Karelia", or in some English-language sources, "White Sea Karelia", is used) is the northern part of East Karelia and Olonets Karelia is the southern part.