A raion (or rayon) is a type of administrative unit of several post-Soviet countries (such as part of an oblast). The term, which is from French "rayon" (meaning "honeycomb, department"), describes both a type of a subnational entity and a division of a city, and is commonly translated in English as "district". The term "raion" also can be used simply as a second degree of administrative division without anything to do with ethnicity or nationality. A raion is usually an administrative entity as subdivided two steps below the national level. However, in smaller countries, it could be the primary level of administrative division.

The word "raion" (or "rayon") is often used in translated form: Russian and Ukrainian: райо́н, Azerbaijani: rayon; Belarusian: раён; Georgian: რაიონი, raioni; Latvian: rajons; Lithuanian: rajonas; Romanian: raion.

In the Soviet Union, raions were administrative divisions created in the 1920s to reduce the number of territorial divisions inherited from the Russian Empire and to simplify their bureaucracies. The process of conversion to the system of raions was called raionirovanie ("regionalization"). It was started in 1923 in the Urals, North Caucasus, and Siberia as a part of the Soviet administrative reform and continued through 1929, by which time the majority of the country's territory was divided into raions instead of the old volosts and uyezds.