Yuliy Osipovich Tsederbaum (Russian: Юлий Осипович Цедербаум [ˈjʉlʲɪj ˈosʲɪpəvʲɪtɕ tsɨdʲɪrˈbaʊm]; 24 November 1873 – 4 April 1923), better known as Julius Martov (Юлий Мартов [ˈmartəf] ), was a Russian revolutionary and a leader of the Mensheviks, the minority faction of the Russian Social Democratic Labour Party (RSDLP). A close friend and collaborator of Vladimir Lenin in the early years of their revolutionary careers, he became his chief rival after the RSDLP split at its Second Congress in 1903.
Born into a middle-class, assimilated Jewish family in Constantinople, Martov became a Marxist activist in the Russian Empire in the early 1890s. With Lenin, he co-founded the League of Struggle for the Emancipation of the Working Class in 1895. Both were arrested shortly after and exiled to Siberia. After his exile, Martov joined Lenin and Georgy Plekhanov in founding the party newspaper Iskra, which became the primary organ of the RSDLP. At the Second Party Congress, Martov's proposal for the definition of party membership, which was broader and more inclusive than Lenin's, was passed. However, after several delegates walked out, Lenin's faction won a vote on the composition of the party's Central Committee, leading to the historic split between Lenin's Bolsheviks ("majority-ites") and Martov's Mensheviks ("minority-ites").