The Russian Social Democratic Labour Party, or RSDLP (Росси́йская Социа́л-Демократи́ческая Рабо́чая Па́ртия = РСДРП), also known as the Russian Social Democratic Workers' Party, was a revolutionary socialist Russian political party formed in 1898 in Minsk to unite the various revolutionary organisations into one party. The RSDLP later split into Bolshevik and Menshevik factions, with the Bolsheviks eventually becoming the Communist Party of the Soviet Union.

It was not the first Russian Marxist group; the Group for the Liberation of Labour was formed in 1883. At the first party congress in 1898, all nine delegates were arrested. The RSDLP was created to oppose narodnichestvo (наро́дничество), revolutionary populism, the program of the Social Democrats (SDs), who later joined the Socialist-Revolutionary Party (SRs; Esers, эсе́ры). The RSDLP program was based on the theories of Karl Marx and Friedrich Engels - that, despite Russia's agrarian nature, the true revolutionary potential lay with the industrial working class.

Before the Second Congress, a young intellectual called Vladimir Ilyich Ulyanov (Влади́мир Ильи́ч Улья́нов) joined the party, better known by his pseudonym - Lenin (Ле́нин). In 1902 he had published What is to be Done?, outlining his view of the party's task and methodology - to form 'the vanguard of the proletariat' needed a disciplined, centralised party of committed activists.

In 1903, the Second Congress of the party met in Belgium to attempt to create a united force. At the congress, the party split into two irreconcilable factions on November 17: the Bolsheviks (большеви́к; from Bolshinstvo - Russian for "majority"), headed by Lenin, and the Mensheviks (меньшеви́к; from Menshinstvo - Russian for "minority"), headed by Julius Martov. Confusingly, the Mensheviks were actually the larger faction, however the names Menshevik and Bolshevik were taken from a vote held at the 1903 party congress for the editorial board of the party journal, Iskra, with the Bolsheviks being the majority and the Mensheviks being the minority. These were the names used by the factions for the rest of the party congress which debated Lenin's proposals on party organisation and these are the names retained after the split at the 1903 congress, even though Lenin's faction ended up in the minority and remained smaller than the Mensheviks until the Russian Revolution.

It was Lenin's position on democratic centralism and on restricting party membership that caused the split. Lenin argued that creating a successful revolution required that party membership be limited only to professional full-time revolutionaries; whereas the Mensheviks favored an open membership policy. Despite a number of attempts at reunification, the split proved permanent.

The SDs boycotted elections to the First Duma (April-July 1906), but were represented in the Second Duma (February-June 1907). With the SRs, they held 83 seats. The Second Duma was dissolved on the pretext of the discovery of an SD conspiracy to subvert the army. Under new electoral laws, the SD presence in the Third Duma (1907-12) was reduced to 19. From the Fourth Duma (1912-17), the SDs were finally and fully split. The Mensheviks had five members in the Duma and the Bolsheviks had seven, including Roman Malinovski , who was later uncovered as an Okhranka agent.

The Bolsheviks seized power during the Russian Revolution and, in 1918, changed their name to the Communist Party. They banned the Mensheviks after the Kronstadt Uprising of 1921.