The Rex Popular Front, or simply Rex, was a far-right Catholic authoritarian and corporatist political party active in Belgium from 1935 until 1945. The party was founded by the journalist Léon Degrelle. It advocated Belgian unitarism and royalism. Initially, the party ran in both Flanders and Wallonia, but it never achieved much success outside Wallonia and Brussels. Its name was derived from the Roman Catholic journal and publishing company Christus Rex.

Léon Joseph Marie Ignace Degrelle was a Belgian Walloon politician and Nazi collaborator. He rose to prominence in Belgium in the 1930s as the leader of the Rexist Party (Rex). During the German occupation of Belgium during World War II, he enlisted in the German army and fought in the Walloon Legion on the Eastern Front. After the collapse of the Nazi regime, Degrelle escaped and went into exile in Francoist Spain, where he remained a prominent figure in neo-Nazi politics.