The Walloon Legion (French: Légion Wallonie, pronounced [leʒjɔ̃ walɔni], lit. 'Wallonia Legion') was a unit of the German Army (Wehrmacht) and later of the Waffen-SS recruited among French-speaking collaborationists in German-occupied Belgium during World War II. It was formed in the aftermath of the German invasion of the Soviet Union and fought on the Eastern Front alongside similar formations from other parts of German-occupied Western Europe.

Established in July 1941, the Walloon Legion was envisaged by Léon Degrelle's Rexist Party as a means of demonstrating its loyalty and political indispensability in German-occupied Belgium where it had been largely ignored since the German invasion of May 1940. A similar formation had already been created by Flemish collaborators as the Flemish Legion, preventing Degrelle from being able to establish the "Belgian Legion" he had originally intended. The formation, initially part of the German Army, was officially designated Infantry Battalion 373 (Infanterie Bataillon 373). Degrelle himself enlisted and increasingly saw the unit as a more important political vehicle than the Rexist Party. It participated in fighting on the Eastern Front from February 1942 but struggled to find sufficient recruits in Belgium to replace its persistently heavy losses.

The unit was integrated into the Waffen-SS in June 1943 as the SS Assault Brigade Wallonia (SS-Sturmbrigade Wallonien) and was almost destroyed by Soviet forces in the Korsun–Cherkassy Pocket in February 1944. It expanded slightly after the Allied Liberation of Belgium in September 1944 as Belgian, French, and Spanish collaborators were drafted into the unit. It was upgraded to the notional status of a division and re-designated as the SS Volunteer Grenadier Division Wallonia (SS-Freiwilligen-Grenadier-Division Wallonien) in October 1944. After heavy losses during the 1945 retreats, its remaining personnel surrendered to British forces in April 1945.

At the time of the German invasion in May 1940, Belgium had several political parties that were broadly sympathetic to the authoritarian and anti-democratic ideals represented by Nazi Germany. In Wallonia and Brussels, the largest of these groups was the Rexist Party, led by Léon Degrelle. This had originated as a faction of the mainstream Catholic Block, but split in 1935 to form an independent populist party. Ideologically, Rex supported Belgian nationalism, but its support for corporatism and anti-communism made it sympathetic towards aspects of Nazi ideology. It achieved some early success, peaking in the 1936 Belgian general election in which it received 11.5 percent of the national vote. In spite of this, the party experienced a rapid decline in the years before the German invasion and polled below five percent in the 1939 elections and remained marginal.

After the Belgian surrender on 28 May 1940, the German Military Administration in Belgium and Northern France governed the occupied country. As part of its strategy of indirect rule, the administration preferred to work with established Belgian political and social elites, largely ignoring fringe political groups such as the Rexists.

In order to acquire more influence and German support, Rex attempted to bring itself closer to the occupation authorities. On 1 January 1941, Degrelle announced Rex's total support for the occupation authorities and for the policy of collaborationism. After the German invasion of the Soviet Union on 22 June 1941, it embraced the idea of raising a military unit, seen as "a political opportunity to increase the importance of their movements and eliminate political competition". At the same time, the Flemish National League (Vlaamsch Nationaal Verbond, VNV), a Flemish nationalist and rival authoritarian party in Flanders, also announced its intention to form a "Flemish Legion" to fight in the German Army in the Soviet Union. This move, combined with the Germans' favourable stance towards the VNV, meant that it would not be possible to realise Rex's preferred option of a national "Belgian Legion" on the Eastern Front.

In July 1941, Rex announced that it would raise a unit of volunteers of its own, dubbed the Wallonia Free Corps (Corps Franc Wallonie) or Walloon Legion (Légion Wallonie). Unlike comparable Flemish and Dutch units, the Walloon Legion was established within the German Army (Wehrmacht) because Walloons were not considered sufficiently "Germanic" by Nazi racial theorists to be allowed into the Waffen-SS. Recruitment initially met with little success, leading Degrelle personally to volunteer for the unit as a private as a publicity stunt. In total, some 850 men had volunteered by August 1941, bringing the unit up to the strength of a battalion. Officially designated as Infantry Battalion 373 (Infanterie Bataillon 373), it was sent for training in Meseritz in Germany. As part of Degrelle's notion of an expanded Burgundian-style Belgium, the unit adopted the Cross of Burgundy as its insignia.

Most of the Legion's initial volunteers were Rexist cadres and many had been part of the Combat Formations (Formations de Combat) which served as the party's paramilitary wing. In propaganda, Rex emphasised the anti-communist dimension of the German war effort and argued that collaboration was compatible with Belgian patriotism. The unit encountered various internal problems with some volunteers being unwilling to swear personal allegiance to Adolf Hitler and others being classed as medically unfit; almost a third of the volunteers were repatriated before October 1941. Over the winter of 1941–1942, it participated in training and security operations near Donetsk in Ukraine.