Olympique de Marseille (French: [ɔlɛ̃pik də maʁsɛj], locally [olɛ̃ˈpikə də maχˈsɛjə]; Occitan: Olimpic de Marselha, pronounced [ulimˈpi de maʀˈsejɔ]), also known simply as Marseille, or by the abbreviation OM (IPA: [o.ɛm], locally [oˈɛmə]), is a French professional football club based in Marseille which competes in Ligue 1, the top flight of French football. Founded in 1899, OM has won 28 domestic trophies: 9 Ligue 1 titles, 10 Coupe de France, 3 Coupe de la Ligue, and 3 Trophée des Champions. Continentally, the club holds a joint national record of one UEFA Champions League and a joint national record of one UEFA Intertoto Cup. Additionally, Marseille has played in three UEFA Europa League finals. In 1993, coach Raymond Goethals led the team to become the first French club to win the UEFA Champions League, defeating Milan 1–0 in the final, the first under the UEFA Champions League branding of the tournament. In 2010, Marseille won its first Ligue 1 title in 18 years under the management of former club captain Didier Deschamps.

Marseille's home ground is the 67,394-capacity Stade Vélodrome in the southern part of the city, where they have played since 1937. The stadium underwent renovations between 2011 and 2014, increasing its capacity to 67,000 in preparation for France's hosting of UEFA Euro 2016. The club has a large fan-base, having regularly averaged the highest attendance in French football. Marseille's average home gate for the 2018–19 season was 50,361, the highest in Ligue 1. The club have a long-standing rivalry with Paris Saint-Germain, against whom they contest Le Classique.

In 1997, Marseille was purchased by Franco-Swiss businessman Robert Louis-Dreyfus. Following his death in 2009, his widow Margarita became the club's majority shareholder in 2010. American businessman Frank McCourt bought 95% of the club's stake in 2016, and appointed businessman Jacques-Henri Eyraud as the club president, later replaced by Pablo Longoria in 2021.

Marseille was listed in the Deloitte Football Money League at the end of the 2023–2024 season, making it the football club with the 19th highest revenue in the world, approximated at €287 million.

In 2023, Marseille was placed twenty-eighth in the global ranking drawn up by the British consultancy organisation Brand Finance in terms of brand power, where it was rated with a credit rating AA (\"very strong\") with a score of 71.3 out of 100, as well as twenty-eighth in terms of brand value (€173 million) and twenty-eighth by enterprise value (€458 million as of 2023).