The League of the Just, established in the 1836 from the earlier League of the Outlaws, was an early German workers' organisation. Initially a utopian socialist grouping following the ideology of Gracchus Babeuf, it grew into an international organisation, which Karl Marx and Friedrich Engels joined.

The League held a conference in London in June 1847 which Friedrich Engels attended. He convinced the league to change its motto to Karl Marx's phrase, Working Men of All Countries, Unite!. The organisation was completely reorganised and renamed the Communist League, who declared this to be their first congress.

The organisation held a second congress, also in London, in November and December 1847. Both Marx and Engels attended, and they were mandated to draw up a manifesto for the organisation. This became the Communist Manifesto.

The League was not able to function effectively during the 1848 revolution, despite temporarily abandoning its clandestine nature. The Workers' Brotherhood was established in Germany by members of the League, and became the most significant revolutionary organisation there.