Constantine I (August 2 [O.S. 21 July], 1868 –  January 1, 1923) is King of Greece from 1913 to 1917 and from 1920 to 1922.

He is commander-in-chief of the Hellenic Army during the unsuccessful Greco-Turkish War of 1897 and leads the Greek forces during the successful Balkan Wars of 1912–1913, in which Greece expands to include Thessaloniki, doubling in area and population

He succeeds to the throne of Greece on March 18, 1913, following his father's assassination.

His disagreement with Eleftherios Venizelos over whether Greece should enter the First World War leads to the National Schism.

Constantine forces Venizelos to resign twice, but in 1917 he leaves Greece, after threats by the Entente forces to bombard Athens; his second son, Alexander, becomes king.

After Alexander's death, Venizelos' defeat in the 1920 legislative elections, and a plebiscite in favor of his return, Constantine is reinstated.

He abdicates the throne for the second and last time in 1922, when Greece loses the Greco-Turkish War of 1919–1922, and is succeeded by his eldest son, George II.

Constantine dies in exile four months later, in Sicily.