The Communist Party of Germany (German: Kommunistische Partei Deutschlands, pronounced [kɔmuˈnɪstɪʃə paʁˈtaɪ ˈdɔʏtʃlants] (listen), KPD [kaːpeːˈdeː] (listen)) was a major political party in the Weimar Republic between 1918 and 1933, an underground resistance movement in Nazi Germany, and a minor party in West Germany in the postwar period until it was banned by the Federal Constitutional Court in 1956.

Founded in the aftermath of the First World War by socialists who had opposed the war, the party joined the Spartacist uprising of January 1919, which sought to establish a soviet republic in Germany. After the defeat of the uprising, and the murder of KPD leaders Rosa Luxemburg, Karl Liebknecht and Leo Jogiches, the party temporarily steered a more moderate, parliamentarian course under the leadership of Paul Levi. During the Weimar Republic period, the KPD usually polled between 10 and 15 percent of the vote and was represented in the national Reichstag and in state parliaments. Under the leadership of Ernst Thälmann from 1925 the party became thoroughly Stalinist and loyal to the leadership of the Soviet Union, and from 1928 it was largely controlled and funded by the Comintern in Moscow. Under Thälmann's leadership the party directed most of its attacks against the Social Democratic Party of Germany, which it regarded as its main adversary and referred to as "social fascists"; the KPD considered all other parties in the Weimar Republic to be "fascists".

The KPD was banned in the Weimar Republic one day after the Nazi Party emerged triumphant in the German elections in 1933. It maintained an underground organization in Nazi Germany, and the KPD and groups associated with it led the internal resistance to the Nazi regime, with a focus on distributing anti-Nazi literature. The KPD suffered heavy losses between 1933 and 1939, with 30,000 communists executed and 150,000 sent to Nazi concentration camps.

The party was revived in divided postwar West and East Germany and won seats in the first Bundestag (West German Parliament) elections in 1949, but its support collapsed following the establishment of the German Democratic Republic in the former Soviet Occupation Zone in the east. The KPD was banned as extremist in West Germany in 1956 by the Federal Constitutional Court. In 1969, some of its former members founded an even smaller fringe party, the German Communist Party (DKP), which remains legal, and multiple tiny splinter groups claiming to be the successor to the KPD have also subsequently been formed.

In East Germany, the party was merged, by Soviet decree, with remnants of the Social Democratic Party to form the Socialist Unity Party (SED) which ruled East Germany from 1949 until 1989–1990; the merger was opposed by many Social Democrats, many of whom fled to the western zones. After the fall of the Berlin Wall, reformists took over the SED and renamed it the Party of Democratic Socialism (PDS); in 2007 the PDS subsequently merged with the SPD splinter faction WASG to form Die Linke.