In Marxist theory, the Lumpenproletariat (German: [ˈlʊmpn̩pʁoletaʁi̯ˌaːt] ^ⓘ; /ˌlʌmpənproʊlɪˈtɛəriət/) is the underclass devoid of class consciousness. Karl Marx and Friedrich Engels coined the word in the 1840s and used it to refer to the unthinking lower strata of society exploited by reactionary and counter-revolutionary forces, particularly in the context of the revolutions of 1848. They dismissed the revolutionary potential of the Lumpenproletariat and contrasted it with the proletariat. Among other groups, criminals, vagabonds, and prostitutes are usually included in this category.

The Social Democratic Party of Germany made wide use of the term by the turn of the 20th century. Vladimir Lenin and Leon Trotsky followed Marx's arguments and dismissed the group's revolutionary potential, while Mao Zedong argued that proper leadership could harness it. The word Lumpenproletariat, popularized in the West by Frantz Fanon's The Wretched of the Earth in the 1960s, has been adopted as a sociological term. However, what some consider to be its vagueness and its history as a term of abuse has led to some criticism. Some revolutionary groups, most notably the Black Panther Party and the Young Lords, have sought to mobilize the Lumpenproletariat.

Karl Marx and Friedrich Engels are generally considered to have coined the term Lumpenproletariat. It is composed of the German word Lumpen, which is usually translated as "ragged" and prolétariat, a French word adopted as a common Marxist term for the class of wage earners in a capitalist system. Hal Draper argued that the root is lump ("knave"), not lumpen. Bussard noted that the meaning of lump shifted from being a person dressed in rags in the 17th century to knavery in the 19th century.

The American Heritage Dictionary of the English Language defines it as "the lowest stratum of the proletariat. Used originally in Marxist theory to describe those members of the proletariat, especially criminals, vagrants, and the unemployed, who lacked awareness of their collective interest as an oppressed class." In modern usage, it is commonly defined to include the chronically unemployed, the homeless, and career criminals.

In English translations of Marx and Engels, lumpenproletariat has sometimes been rendered as "social scum", "dangerous classes", "ragamuffin", and "ragged-proletariat". It has been described by some scholars and theorists, as well as the Soviet nomenclature, as a declassed (déclassé) group. The term "underclass" is considered to be the modern synonym of lumpenproleteriat. Scholars note its negative connotations. Economist Richard McGahey, writing for the New York Times in 1982, noted that it is one of the older terms in a "long line of labels that stigmatize poor people for their poverty by focusing exclusively on individual characteristics." He listed the following synonyms: "underclass", "undeserving poor", and "culture of poverty". Another synonym is "riff-raff". The word is used in some languages as a pejorative. In English, it may be used in an informal disapproving manner to "describe people who are not clever or well educated, and who are not interested in changing or improving their situation."

According to historian Robert Bussard, Karl Marx and Friedrich Engels viewed the lumpenproletariat as:

They used the term exclusively with negative connotations, although their works lack a "consistent and clearly reasoned definition" of the term. They used the term in various publications "for diverse purposes and on several levels of meaning."

Hal Draper suggested that the concept has its roots in Young Hegelian thought and possibly in G.W.F. Hegel's Elements of the Philosophy of Right. While Bussard believes that the idea was "at one and the same time, a hybrid of new social attitudes which crystallised in France, England and Germany, as well as an extension of more traditional, pre-nineteenth-century views of the lower classes." Bussard noted that they often used the term as a "kind of sociological profanity" and contrasted between it and "working and thinking" proletariat. According to Michael Denning, by identifying the lumpenproletariat, "Marx was combating the established view that the entire working class was a dangerous and immoral element. He drew a line between the proletariat and the lumpenproletariat to defend the moral character of the former."