Georgios Siantos (nicknames: Geros "Old man", Theios "Uncle"; Greek: Γεώργιος "Γιώργης" Σιάντος; 1890 – May 20, 1947) was a prominent figure of the Communist Party of Greece (Greek: Κομμουνιστικό Κόμμα Ελλάδας, Kommunistiko Komma Elladas, KKE) who served as acting general secretary of the party and as a leader of the National Liberation Front (EAM)/Greek People's Liberation Army (ELAS) Resistance movement during the German occupation of Greece in World War II.
Siantos was born in Karditsa, Thessaly in 1890; his parents were farmers. When he graduated from primary school, Siantos started working as a tobacco worker. He served in the Greek infantry as a sergeant during the Balkan Wars of 1912-1913 and World War I. By 1916 he had gone over to the liberals of Eleftherios Venizelos. Siantos also became involved in the labor movement, and, soon after his demobilization in 1920, he was elected president of the trade union of the tobacco workers in Karditsa.
In 1922, Siantos was elected Secretary General of the Greek Confederacy of Tobacco Workers, while, during the same period, he became a member of the Socialist Labour Party of Greece (SEKE, Greek: Σοσιαλιστικό Εργατικό Κόμμα Ελλάδας, Sosialistiko Ergatiko Komma Elladas), the forerunner of KKE. In 1927 Siantos became a member of the central committee of the politburo of the party. During the internal ideological struggle of 1929-1931, Siantos as a leading member of the "left faction" was called to Moscow, where he accepted the admonitions of Comintern for unity in KKE. After his returning to Greece, Siantos participated in the administration of General Confederation of Greek Workers, and in 1934 he was reelected in the central committee and the politburo of the party.