Robert Siewert (30 December 1887 – 2 November 1973) was a German politician and fought in the German Resistance against National Socialism. He is a survivor of Buchenwald concentration camp, where he helped save the life of Stefan Jerzy Zweig (through causing the death of a Romani boy named Willi Blum), among others.
Siewert was born the son of a carpenter in Schwersenz, Province of Posen, German Empire (today Swarzędz, Poznań County in Poland). He learned the trade of masonry and became a member of the Social Democratic Party (SPD) in 1906. From 1908 to 1915, he worked as a bricklayer in Switzerland, where he got to know Vladimir Lenin and Heinrich Brandler.
Siewert was a soldier during the First World War, serving on the eastern front, while also working for the Spartacist League. In 1918, he was a member of the Soldiers' Council of the 10th Army. After that, he became a member of the Communist Party of Germany (KPD).
In 1919, Siewert was the district political leader in the Ore Mountains and in 1919 and 1920, he was a delegate to the party congress and then secretary to the unification congress when the KPD merged with the USPD. He was elected to the Central Committee at the KPD congresses in 1921 and 1923. In 1922, he was a delegate to the Fourth World Congress of the Communist International and he joined the leadership of the KPD publishers. He became the political leader in Chemnitz in 1923.