Royale Union Saint-Gilloise, abbreviated to Union SG or USG, unofficially simply called Union, is a Belgian football club originally located in the municipality of Saint-Gilles, in Brussels, although since the 1920s it has been based at the Stade Joseph Marien in the neighbouring municipality of Forest. The club is one of the most successful in the history of Belgian football. The club won eleven Belgian championships between 1904 and 1935, making it the most successful Belgian club before World War II. The team colours are blue and yellow and its matricule is 10. The team is traditionally popular with the Flemish- and French-speaking natives of Brussels and both the French and the Flemish speaking free universities. On 13 March 2021, after defeating local rivals R.W.D. Molenbeek, the club was promoted back to the Belgian First Division A, marking its first appearance in top-flight football in 48 years.