Rosa Luxemburg (Polish: Róża Luksemburg [ˈruʐa ˈluksɛmburk] (listen); German: [ˈʁoːza ˈlʊksm̩bʊʁk] (listen); born Rozalia Luksenburg; 5 March 1871 – 15 January 1919) was a Polish and naturalised-German revolutionary socialist, Marxist philosopher and anti-war activist.

Born and raised in a secular Jewish family in Congress Poland, she became a German citizen in 1897. Successively, she was a member of the Proletariat party, the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL), the Social Democratic Party of Germany (SPD), the Independent Social Democratic Party (USPD), the Spartacus League (Spartakusbund), and the Communist Party of Germany (KPD).

After the SPD supported German involvement in World War I in 1915, Luxemburg and Karl Liebknecht co-founded the anti-war Spartacus League (Spartakusbund) which eventually became the KPD. During the November Revolution, she co-founded the newspaper Die Rote Fahne (The Red Flag), the central organ of the Spartacist movement. Luxemburg considered the Spartacist uprising of January 1919 a blunder, but supported the attempted overthrow of the SPD-ruled Weimar Republic and rejected any attempt at a negotiated solution. Friedrich Ebert's SPD Cabinet crushed the revolt and the Spartakusbund by sending in the Freikorps, government-sponsored paramilitary groups consisting mostly of battle-hardened World War I veterans of the Imperial German Army. Freikorps troops captured and assassinated Luxemburg and Liebknecht during the rebellion.

Due to her pointed criticism of both the Leninist and the more moderate social democratic schools of Marxism, Luxemburg has always had a somewhat ambivalent reception among scholars and theorists of the political left. Nonetheless, Luxemburg and Liebknecht were extensively idolised as communist martyrs by the East German communist government. The German Federal Office for the Protection of the Constitution (BVS) asserts that idolization of Luxemburg and Liebknecht is an important tradition of the 21st-century German far-left. Despite her own Polish nationality and strong ties to Polish culture, opposition from the PPS due to her stance against the 1918 independence of the Second Polish Republic and later criticism from Stalinists have made her a controversial historical figure in the present-day political discourse of the Third Polish Republic.