Georgi Valentinovich Plekhanov (Russian: Георгий Валентинович Плеханов [ɡʲɪˈorɡʲɪj vəlʲɪnʲˈtʲinəvʲɪtɕ plʲɪˈxanəf] ^ⓘ; 11 December [O.S. 29 November] 1856 – 30 May 1918) was a Russian Marxist theorist, philosopher, and revolutionary. After beginning his revolutionary career as a populist, in 1883 Plekhanov established the Emancipation of Labour group, the first Russian Marxist political organisation. He is widely regarded as the "father of Russian Marxism", and his theoretical works were instrumental in converting a generation of revolutionaries, including Vladimir Lenin, to the cause.

Plekhanov was a prominent leader in the Russian Social Democratic Labour Party (RSDLP) and the Second International. In 1900, he collaborated with Lenin in founding the party newspaper Iskra, and at the party's Second Congress in 1903, initially sided with Lenin's Bolshevik faction. However, he soon broke with the Bolsheviks over their organisational principles, which he criticised as overly centralist, and became a leading figure in the opposing Menshevik faction. During the 1905 Russian Revolution, Plekhanov maintained that Russia was only ready for a bourgeois-democratic revolution and argued against what he saw as premature attempts to seize power by the proletariat.

During World War I, Plekhanov adopted a staunchly nationalist position, "defensism", in support of the Allied cause, a stance that separated him from most international socialists. He returned to Russia after the 1917 February Revolution and supported the Provisional Government. He was a fierce opponent of the Bolsheviks, denouncing their leader Lenin and warning that their seizure of power in the October Revolution would be a disaster for the country. Plekhanov died of tuberculosis in Finland the following year.

Despite his political opposition to the Bolsheviks, Plekhanov was held in high esteem by Lenin and was posthumously enshrined in the Soviet Union as a founding father of Russian Marxism. His contributions to philosophy, historical materialism, and aesthetics made him one of the most important Marxist thinkers of his era. His legacy remains contested, with some viewing him as a democratic, orthodox Marxist alternative to Leninism, while others focus on his theoretical groundwork that paved the way for the Bolsheviks.

Georgi Valentinovich Plekhanov was born on 11 December [O.S. 29 November] 1856 in the Russian village of Gudalovka in the Tambov Governorate. He was born into a noble family of Volga Tatar ancestry. Plekhanov's father, Valentin, was a member of the lower stratum of the landed gentry. He possessed about 270 acres of land and some 50 serfs. Valentin was a military man who had served in the Crimean War and in the suppression of the 1863 Polish uprising. In later life, he managed his estate and was described by his children and former serfs as severe and sometimes violent. Despite his opposition to the emancipation reform of 1861, which freed the serfs and deprived him of half his estate, he attempted to adapt to the new conditions of capitalism. Valentin sought to instill in his children the values of manliness, courage, self-reliance, and activity, and discouraged idleness.

Plekhanov's mother, Maria Feodorovna, was distantly related to the famed literary critic Vissarion Belinsky. She was Valentin's second wife; they married in 1855, and Georgi was the first of their five children. Maria was a gentle and compassionate woman who undertook the early education of her children. She encouraged Georgi's intellectual interests, teaching him to read at an early age. Plekhanov later recalled that it was she who instilled in him a sense of altruism and justice. His relationship with his mother was warm, while his relationship with his father was more reserved.

When he was ten years old, Plekhanov's family enrolled him in the Voronezh Military Academy. At the academy, he was influenced by his teacher, N. F. Bunakov, a proponent of liberal pedagogical ideas who instilled in Plekhanov a love of literature and a sense of responsibility to the Russian people. Bunakov introduced him to the writings of radical literary critics like Belinsky and Nikolay Chernyshevsky, giving the young Plekhanov his first acquaintance with the ideas of the intelligentsia. Under the influence of the radical poet Nikolay Nekrasov, he developed a deep sympathy for the suffering of the Russian people. During his time at the academy, Plekhanov also broke with his mother's Orthodox faith and became an atheist, challenging the priest who taught sacred law with probing questions.

In 1873, after graduating from the academy, Plekhanov registered at the Konstantinovskoe Military School in Saint Petersburg. While in the capital, his interest in military drills waned as he spent more time with Russian literature and literary criticism. He soon decided against a military career and, after only one semester, withdrew from the school to prepare for the entrance examinations to the Mining Institute. Plekhanov's decision to pursue a career in mining engineering rather than social studies was likely influenced by the radical spirit of the 1860s and 1870s, which was characterized by utilitarianism, positivism, and a high regard for the natural sciences.