The Social Democratic Workers' Party of Germany, in German Sozialdemokratische Arbeiterpartei Deutschlands, SDAP, was a German left-wing political party founded in 1869 in Eisenach, Germany by, among others, Wilhelm Liebknecht and August Bebel.

The party included some former members of the General German Workers' Association (ADAV) of Ferdinand Lassalle who had died five years earlier. They were known as the Eisenacher after the town where the party was founded, and their programme was the Eisenacher Programm. Bebel and Liebknecht were influenced by Karl Marx and Friedrich Engels and considered their party "a section of the International Workingmen's Association", sharing its aspirations. The party, which was based mainly in Saxony, demanded the democratisation of the state and of German society.

The ADAV and the SDAP were very similar, and in 1875 the two parties finally merged to form the SAPD, which in turn became today's SPD.