Rosa Luxemburg (/ˈlʌksəmbɜːrɡ/ LUK-səm-burg; Polish: Róża Luksemburg [ˈruʐa ˈluksɛmburk] ^ⓘ; German: [ˈʁoːza ˈlʊksm̩bʊʁk] ^ⓘ; 5 March 1871 – 15 January 1919) was a Polish and naturalised-German Marxist theorist and revolutionary. She was a leading theorist of the Social Democratic Party of Germany (SPD) and later co-founded the anti-war Spartacus League, which evolved into the Communist Party of Germany (KPD). An influential member of the international socialist movement, she is remembered for her writings on imperialism and revolution, and as a champion of socialist democracy who famously stated, "Freedom is always and exclusively freedom for the one who thinks differently."

Born and raised in Russian-ruled Poland to a secular Jewish family, Luxemburg became active in revolutionary politics in her youth. She co-founded the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL), a party that rejected Polish nationalism in favour of an international class struggle. After moving to Germany in 1898, she became the foremost voice of the SPD's revolutionary wing. In her 1900 pamphlet Social Reform or Revolution?, she defended the necessity of revolution against the reformist theories of Eduard Bernstein, arguing that the struggle for reforms was a means to an end, not an end in itself. Inspired by the 1905 Russian Revolution, she developed a theory of the mass strike as the proletariat's most important revolutionary tool, emphasizing the spontaneous creativity of the working class.

As World War I approached, Luxemburg's anti-militarist and anti-imperialist convictions brought her into increasing conflict with the SPD leadership. In her major work, The Accumulation of Capital (1913), she argued that capitalism's need to expand into non-capitalist regions to survive was the driving force behind imperialism. She fiercely condemned the party's support for the war, and was imprisoned for most of the conflict. From prison, she wrote the influential Junius Pamphlet, which declared the war a betrayal of the working class and popularized the phrase "socialism or barbarism" to describe the choice facing humanity. While she celebrated the Russian Revolution of 1917, in a posthumously published manuscript she offered a sharp critique of the Bolsheviks' authoritarian policies, defending democratic freedoms and the need for a revolution rooted in mass participation. Her writings on the Russian Revolution were later viewed by some as a "premature critique" of Stalinism.

Released from prison during the German Revolution of 1918–1919, Luxemburg co-founded the KPD and became a central figure in the January 1919 Spartacist uprising in Berlin. After the uprising was crushed by the Freikorps, a government-sponsored paramilitary group, Luxemburg and her comrade Karl Liebknecht were captured and summarily executed. Following her death, Luxemburg became a heroine and martyr for Marxists. Her legacy has been a subject of intense debate, with her emphasis on spontaneity and democracy celebrated by many on the left—particularly by the New Left and those in the libertarian socialist tradition—but sharply criticized by the Stalinist tradition, which denounced "Luxemburgism" as a heresy.

Rozalia Luksenburg was born on 5 March 1871 in Zamość, a town in the Russian-controlled Congress Poland. She was the fifth and youngest child of a moderately well-off, assimilated, secular Jewish family. Her father, Elias (or Eduard) Luxemburg, was a timber merchant with a German education who was sympathetic to the Polish national movement. Her mother, Lina Löwenstein, was descended from a long line of rabbis. The family was committed to the values of the Haskalah, the Jewish enlightenment movement, and embraced progressive European culture. The family had largely abandoned a conscious Jewish life; they spoke Polish and German at home, and Rosa, like her four siblings, received a secular education. She had a complex relationship with her Jewish identity; while proud of her heritage, she rejected any specific Jewish political cause, insisting that the suffering of Jews was no worse than the often murderous oppression of other peoples by European imperialism. She stated later in life: "I have no special corner of my heart for the [Jewish] ghetto. I feel at home in the entire world wherever there are clouds and birds and human tears." As such, the Jewish socialist Bund held no attraction for her, and her concerns consistently transcended nationality.

In 1873, the family moved to Warsaw to seek better business opportunities, a better education for their children, and to escape the ever-present anti-Jewish sentiment and Orthodox-Hasidic dominance in Zamość. At age five, Luxemburg developed a hip disease, probably a congenital hip dislocation. It was misdiagnosed as tuberculosis and resulted in a year-long confinement in a cast, during which she taught herself to read and write. The illness left her with a permanent limp, a condition that deeply affected her and which she later blamed her parents for not detecting earlier.

In 1880, she enrolled at the Second Girls' High School in Warsaw. Admission for Jewish students was subject to a quota, a humiliation which intensified her sense of being an outsider. The school was an instrument of Russification, forbidding the use of Polish. The 1881 Warsaw pogrom, which her family experienced, left her with a permanent fear of mob violence. In response to this frightening reality, she found refuge in the poetry of the Polish Romantic Adam Mickiewicz, whose appeal to rebellion and dreams of universal freedom became a source of inspiration. During this time, while still in her teens, Luxemburg became involved in clandestine student circles associated with the revolutionary Proletariat party. This was the first Polish socialist party, founded in 1882 by Ludwik Waryński. The party was internationalist in outlook, prioritising the economic struggle of the working class and opposing "romantic" ideas such as national liberation, which it believed would deflect or compromise class consciousness. By her final year, Luxemburg was known to the authorities as a politically active and rebellious student, and she was denied the gold medal for academic achievement which her scholastic merits had earned.

After graduating in 1887, she continued her revolutionary activities. She was part of a cell of the "Second Proletariat", one of the successor groups to the original party which had been broken up by arrests in the mid-1880s. By 1889, threatened with arrest, she was smuggled out of Poland with the help of her mentor, Marcin Kasprzak. According to one account, she was hidden under straw in a peasant's cart and taken across the border by a Catholic priest who had been told she was a Jewish girl fleeing to be baptized.