A Reichsgau (plural Reichsgaue) was an administrative subdivision created in a number of areas annexed by Nazi Germany between 1938 and 1945.

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The term was formed from the words Reich (realm, empire) and Gau, the latter a deliberately medieval-sounding word with a meaning approximately equivalent to shire. The Reichsgaue were an attempt to resolve the administrative chaos resulting from the mutually overlapping jurisdictions and different boundaries of the NSDAP Party Gaue, placed under a Party Gauleiter, and the federal states, under a Reichsstatthalter responsible to the Ministry of the Interior (in the Prussian provinces, the equivalent post was that of Oberpräsident). Interior Minister Wilhelm Frick had long desired to streamline the German administration, and the Reichsgaue were the result: the borders of party Gaue and those of the federal states were to be identical, and the party Gauleiter also occupied the post of Reichsstatthalter. Rival interests and the influence the Gauleiter wielded with Hitler prevented any reform from being undertaken in the "Old Reich" (German: Altreich), which meant Germany in its borders of 1937 before the annexation of other territories like Austria, the Sudetenland, and Bohemia, and the Reichsgau scheme was therefore implemented only in newly-acquired territories.

There were several Reichsgaue:

• East March (German: Ostmark) formed from the formerly independent Austria
• Sudetenland, formed from a substantial part of the German-speaking outer rim areas of the former Czechoslovakia occupied in 1938
• Danzig-West Prussia (German: Danzig-Westpreußen) and Wartheland, formed from the Free City of Danzig and areas annexed from Poland

The East March was subsequently subdivided into seven smaller Reichsgaue, generally coterminous with the former Austrian Länder (federal provinces).

• Administrative divisions of Nazi Germany
• Polish areas annexed by Nazi Germany