Royale Union Saint-Gilloise ([y.njɔ̃ sɛ̃.ʒil.waz] ), also known as Union Saint-Gilloise, Union SG or simply Union, is a Belgian professional football club from Brussels based in Forest. Founded in 1897 in the neighbouring Saint-Gilles, the club has played its home matches at the historic Joseph Marien Stadium since the 1920s. One of the most successful clubs in Belgian football history, Union won eleven national titles between 1904 and 1935, dominating the domestic game before World War II. From 1933 to 1935, they went 60 league matches unbeaten—a national record that still stands.

After a long decline that saw the club fall as low as the fourth tier, Union experienced a resurgence in the 2020s following a takeover by British businessman Tony Bloom, also chairman of Brighton & Hove Albion. In 2021, they returned to the Belgian Pro League after 48 years and finished top of the regular-season table in their first campaign back—a first for a newly promoted side. Though they narrowly missed out on the championship in 2022, 2023, and 2024, Union clinched their twelfth league title in 2025, ending a 90-year championship drought—one of the longest in European football.

Alongside their domestic revival, the club has re-established a presence in European competition. They reached the quarter-finals of the 2022–23 UEFA Europa League and have qualified for the UEFA Champions League league phase for the first time. Known for their loyal fan base, inclusive stadium culture, and progressive values, Union Saint-Gilloise has become one of Belgian football's most compelling modern success stories.