George I (Greek: Γεώργιος Αʹ, Βασιλεύς των Ελλήνων, Geórgios Αʹ, Vasiléfs ton Ellínon; born Prince William of Schleswig-Holstein-Sonderburg-Glücksburg) was King of Greece from 1863 until his assassination in 1913.
Originally a Danish prince, George was born in Copenhagen, and seemed destined for a career in the Royal Danish Navy. He was only 17 years old when he was elected king by the Greek National Assembly, which had deposed the unpopular former king Otto. His nomination was both suggested and supported by the Great Powers: the United Kingdom of Great Britain and Ireland, the Second French Empire and the Russian Empire. He married the Russian grand duchess Olga Constantinovna of Russia, and became the first monarch of a new Greek dynasty. Two of his sisters, Alexandra and Dagmar, married into the British and Russian royal families. King Edward VII and Tsar Alexander III were his brothers-in-law and King George V and Tsar Nicholas II were his nephews.
George's reign of almost 50 years (the longest in modern Greek history) was characterized by territorial gains as Greece established its place in pre-World War I Europe. Britain ceded the Ionian Islands peacefully, while Thessaly was annexed from the Ottoman Empire after the Russo-Turkish War (1877–1878). Greece was not always successful in its territorial ambitions; it was defeated in the Greco-Turkish War (1897). During the First Balkan War, after Greek troops had captured much of Greek Macedonia, George was assassinated in Thessaloniki. Compared to his own long tenure, the reigns of his successors Constantine, Alexander, and George II proved short and insecure.
King George I was styled "Βασιλεύς τῶν Ἑλλήνων" (King of the Hellenes).