The Socialist Workers' Party of Germany (German: Sozialistische Arbeiterpartei Deutschlands, SAPD) was a centrist Marxist political party in Germany. It was formed by a left-wing party with around 20,000 members which split off from the SPD in the autumn of 1931. In 1931, the remnants of USPD merged into the party, and in 1932 some Communist Party dissenters joined the group too, as well as a part from the Communist Party Opposition. Nevertheless, its membership remained small. From 1933, the group's members worked illegally against National Socialism.
In his home town of Lübeck, the young Herbert Karl Frahm, later known as Willy Brandt, joined the SAPD, against the advice of his mentor Julius Leber. In his autobiography, Brandt wrote:
In autumn 1931, Nazis and German nationalists, the SA and the Stahlhelm joined together to form the "Harzburg Front". ... It was just at this time that the left wing of the social democrats split off, as a result of measures connected to organisation and discipline by the party leaders. A few Reichstag assemblymen, a number of active party groups – above all in Saxony – and not least a large proportion of young Socialists followed the people who were calling for the founding of a Socialist Workers' Party.
In 1934, the youth of SAPD took part in the foundation of the International Bureau of Revolutionary Youth Organizations. The congress, which was held in the Netherlands, was broken up by Dutch police. Several SAPD delegates were handed over to German authorities. The congress then re-convened in Lille. Brandt was elected to the Secretariat of the organization, and worked in Norway for the Bureau.
The SAPD was affiliated to the International Revolutionary Marxist Centre, but broke with the main party of that international, the Independent Labour Party, over the question of the united front and popular front.
During the Second World War some SAPD members emigrated to Great Britain and worked for the party there. Many of those became members of the Social Democratic Party of Germany. Therefore the SAPD was not re-founded anew after the Second World War. Willy Brandt eventually became the leader of the Social Democratic Party, one of West Germany's major political parties of the modern era, and was elected Chancellor of Germany.