The proletariat (/ˌproʊlɪˈtɛəriət/; from Latin proletarius 'producing offspring') is the social class of wage-earners, those members of a society whose possession of significant economic value is their labour power (their capacity to work). A member of such a class is a proletarian or a proletaire. Marxist philosophy regards the proletariat under conditions of capitalism as an exploited class⁠ forced to accept meager wages in return for operating the means of production, which belong to the class of business owners, the bourgeoisie.

Karl Marx argued that this capitalist oppression gives the proletariat common economic and political interests that transcend national boundaries, impelling them to unite and to take over power from the capitalist class, and eventually to create a socialist society free from class distinctions.

The proletarii constituted a social class of Roman citizens who owned little or no property. The name presumably originated with the census, which Roman authorities conducted every five years to produce a register of citizens and their property, which determined their military duties and voting privileges. Those who owned 11,000 assēs (coins) or fewer fell below the lowest category for military service, and their children—prōlēs (offspring)—were listed instead of property; hence the name proletarius (producer of offspring). Roman citizen-soldiers paid for their own horses and arms, and fought without payment for the commonwealth, but the only military contribution of a proletarius was his children, the future Roman citizens who could colonize conquered territories. Officially, propertyless citizens were called capite censi because they were "persons registered not as to their property...but simply as to their existence as living individuals, primarily as heads (caput) of a family."

Although explicitly included by name in the Comitia Centuriata (Centuriate Assembly), proletarii were the lowest class, largely deprived of voting rights. Not only did proletarii have less voting "weight" in the various elections, but since voting ran hierarchically starting with the highest social ranks, a majority could be reached early and their votes never even taken. Late Roman historians such as Livy vaguely described the Comitia Centuriata as a popular assembly of early Rome composed of centuriae, voting units representing classes of citizens according to wealth. This assembly, which usually met on the Campus Martius to discuss public policy, designated the military duties of Roman citizens. One of the reconstructions of the Comitia Centuriata features 18 centuriae of cavalry, and 170 centuriae of infantry divided into five classes by wealth, plus 5 centuriae of support personnel called adsidui, one of which represented the proletarii. In battle, the cavalry brought their horses and arms, the top infantry class full arms and armour, the next two classes less, the fourth class only spears, the fifth slings, while the assisting adsidui held no weapons. If unanimous, the cavalry and top infantry class were enough to decide an issue. A deeper reconstruction incorporating social backgrounds found that the senators, the knights, and the first class held 88 out of 193 centuriae, the two lowest propertied classes held only 30 centuriae, but the proletarii held only 1. Musicians, by way of comparison, had more voting power despite far fewer citizens, with 2 centuriae. "[F]or the voting to reach the proletarii a very profound split in the elite and the higher classes was required."

In the early 19th century, many Western European liberal scholars—who dealt with social sciences and economics—pointed out the socio-economic similarities of the modern rapidly growing industrial worker class and the classic proletarians. One of the earliest analogies can be found in the 1807 paper of French philosopher and political scientist Hugues Felicité Robert de Lamennais. Later it was translated to English with the title "Modern Slavery".

Swiss liberal economist and historian Jean Charles Léonard de Sismondi was the first to apply the proletariat term to the working class created under capitalism, and whose writings were frequently cited by Karl Marx. Marx most likely encountered the term while studying the works of Sismondi.

Marx, who studied Roman law at the Friedrich Wilhelm University of Berlin, used the term proletariat in his socio-political theory (Marxism) to describe a progressive working class untainted by private property and capable of revolutionary action to topple capitalism and abolish social classes, leading society to ever higher levels of prosperity and justice.

Marx defined the proletariat as the social class having no significant ownership of the means of production (factories, machines, land, mines, buildings, vehicles) and whose only means of subsistence is to sell their labour power for a wage or salary.