The Reichsgau Flanders (German: Reichsgau Flandern; Dutch: Rijksgouw Vlaanderen) was a short-lived Reichsgau of Nazi Germany established in 1944. It encompassed the present-day Flemish Region in its old provincial borders (in other words, including Comines-Warneton but excluding Voeren). Brussels was also excluded and given its own territorial arrangement.
In spite of this uncompromising attitude at the time, it was decided that the entire area should someday be assimilated into the Third Reich and divided into three new Reichsgaue of a Greater Germanic Reich: Flandern and Brabant for the Flemish territories, and Wallonien for the Walloon parts. Reichsgau Brabant was to be headed by Gauleiter U. van Brusselen. On 12 July 1944, a Reichskommissariat Belgien-Nordfrankreich was established to accomplish precisely this goal, derived from the previous military administration. This step was curiously only taken at the very end of World War II, when Germany's armies were already in full retreat. The new government was already ousted by the Allied advances in Western Europe in September 1944, and the authority of the Belgian government-in-exile was restored. The actual incorporation into the Nazi state of these new provinces therefore only occurred de jure and with its leaders already in exile in Germany. The only place where any notable gain was made in re-establishing Reich authority occurred in parts of southern Wallonia during the Ardennes Campaign. The collaborators merely achieved a Pyrrhic victory since when the Allied tanks had rolled into Belgium several months before this already signalled the end of their personal domains in the Reich. Many of their supporters fled to Germany where they were conscripted into the Waffen-SS to participate in the final military campaigns of the Third Reich.