The political slogan "Workers of the world, unite!" is one of the most famous rallying cries from The Communist Manifesto (1848)^[2][3][4][5] by Karl Marx and Friedrich Engels (German: Proletarier aller Länder vereinigt Euch!, literally "Proletarians of all countries, unite!",^[6] but soon popularised in English as "Workers of the world, unite! You have nothing to lose but your chains!").^[6][1] A variation of this phrase ("Workers of all lands, unite") is also inscribed on Marx's tombstone.^[8] The essence of the slogan is that members of the working classes throughout the world should cooperate to achieve victory in the class conflict.

Five years before The Communist Manifesto, this phrase appeared in the 1843 book The Workers' Union by Flora Tristan.^[9]

The International Workingmen's Association, described by Engels as "the first international movement of the working class" was persuaded by Engels to change its motto from the League of the Just's "all men are brothers" to "working men of all countries, unite!".^[10] It reflected Marx's and Engels' view of proletarian internationalism.

The phrase has overlapping meanings: first, that workers should unite in unions to better push for their demands such as workplace pay and conditions;^[11] secondly, that workers should see beyond their various craft unions and unite against the capitalist system;^[12] and thirdly, that workers of different countries have more in common with each other than workers and employers of the same country.

The phrase was used by the Industrial Workers of the World (IWW) in their publications and songs^[13][14] and was a mainstay on banners in May Day demonstrations. The IWW used it when opposing World War I in both the United States^[14] and Australia.^[15]

The slogan failed miserably during World War I, when "workers of the world" fought each other on the opposite sides. ^[16]

The slogan was the Soviet Union's state motto (Пролетарии всех стран, соединяйтесь!; Proletarii vsekh stran, soyedinyaytes'!) and it appeared in the State Emblem of the Soviet Union. It also appeared on 1919 Russian SFSR banknotes (in Arabic, Chinese, English, French, German, Italian and Russian), on Soviet coins from 1921 to 1934 and in most Soviet newspapers.

Some socialist and communist parties continue using it.^[17] Moreover, it is often chanted during labor strikes and protests.

In the first Swedish translation of The Communist Manifesto, published in 1848, the translator Pehr Götrek substituted the slogan with Folkets röst, Guds röst! (i.e. Vox populi, vox Dei, or "The Voice of the People, the Voice of God"). However, later translations have included the original slogan.^[18]

Among Maoist-oriented groups a variation invented by Vladimir Lenin, "Workers and Oppressed Peoples and Nations of the World, Unite!", is sometimes used. This slogan was the rallying cry of the 2nd Comintern congress in 1920 and denoted the anti-imperialist and anti-colonialist agenda of the Comintern.