Orthodox Marxism is the body of Marxist thought which emerged after the deaths of Karl Marx and Friedrich Engels in the late 19th century, expressed in its primary form by Karl Kautsky. Kautsky's views of Marxism dominated the European Marxist movement for two decades, and orthodox Marxism was the official philosophy of the majority of the socialist movement as represented in the Second International until the First World War in 1914, whose outbreak caused Kautsky's influence to wane and brought to prominence the orthodoxy of Vladimir Lenin. Orthodox Marxism aimed to simplify, codify and systematize Marxist method and theory by clarifying perceived ambiguities and contradictions in classical Marxism.

Orthodox Marxism maintained that Marx's historical materialism was a science which revealed the laws of history and proved that the collapse of capitalism and its replacement by socialism were inevitable. The implications of this deterministic view were that history could not be "hurried" and that politically workers and workers' parties must wait for the material economic conditions to be met before the revolutionary transformation of society could take place. For example, this idea saw the Social Democratic Party of Germany (SPD) adopt a gradualist approach, taking advantage of bourgeois parliamentary democracy to improve the lives of workers until capitalism was brought down by its objective internal contradictions.

The use of "orthodox" to refer to Kautsky's line is primarily to distinguish it from the reformism of Eduard Bernstein. Such "revisionists" were reviled by the orthodox Marxists for breaking with Marx's thought.

The emergence of orthodox Marxism is associated with the latter works of Friedrich Engels, such as the Dialectics of Nature and Socialism: Utopian and Scientific, which were efforts to popularise the work of Karl Marx, render it systematic and apply it to the fundamental questions of philosophy. Daniel De Leon, an early American socialist leader, contributed much to the thought during the final years of the 19th century and the early 20th century. Orthodox Marxism was further developed during the Second International by thinkers such as Georgi Plekhanov and Karl Kautsky in Erfurt Program and The Class Struggle (Erfurt Program).

Karl Kautsky is recognized as the most authoritative promulgator of orthodox Marxism following the death of Engels in 1895. As an advisor to August Bebel, leader of the Social Democratic Party of Germany (SPD) until Bebel's death in 1913 and as editor of Die Neue Zeit from 1883 till 1917, he was known as the "Pope of Marxism". He was removed as editor by the leadership of the SPD when the Independent Social Democratic Party of Germany (USPD) split away from the SPD. Kautskyism, based on his interpretations of Marxism, became a significant ideological current within socialist thought. Kautsky was an outspoken critic of Bolshevism and Leninism, seeing the Bolsheviks (or Communists as they had renamed themselves after 1917) as an organization that had gained power by a coup and initiated revolutionary changes for which there was no economic rationale in Russia.^{[citation needed]} Kautsky was also opposed to Eduard Bernstein's reformist politics in the period 1896–1901.^{[citation needed]}

Menshevism refers to the political positions taken by the Menshevik faction of the Russian Social Democratic Labour Party prior to the October Revolution of 1917. The Mensheviks believed that socialism could not be realized in Russia due to its backwards economic conditions and that Russia would first have to experience a bourgeois revolution and go through a capitalist stage of development before socialism became technically possible and before the working class could develop the class consciousness for a socialist revolution. The Mensheviks were thus opposed to the Bolshevik idea of a vanguard party and their pursuit of socialist revolution in semi-feudal Russia.

The characteristics of orthodox Marxism are:

Orthodox Marxism is contrasted with revisionist Marxism.