George I (Greek: Γεώργιος Α΄, romanized: Geórgios I; 24 December 1845 – 18 March 1913) was King of Greece from 30 March 1863 until his assassination on 18 March 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 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 Grand Duchess Olga Constantinovna of Russia in 1867, 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. Edward VII of the United Kingdom and Alexander III of Russia were his brothers-in-law, and George V of the United Kingdom, Christian X of Denmark, Haakon VII of Norway, and Nicholas II of Russia 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 in 1864, 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 and Epirus, George was assassinated in Thessaloniki.

George was born on 24 December 1845 at his parents' residence the Yellow Palace, an 18th-century town house at 18 Amaliegade, next to the Amalienborg Palace complex in Copenhagen. He was the second son and third child of Prince Christian of Schleswig-Holstein-Sonderburg-Glücksburg and Princess Louise of Hesse-Kassel. He was baptised with the names Christian William Ferdinand Adolph George, and until his accession in Greece, he was known as Prince William, the namesake of both his grandfathers, William, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, and Prince William of Hesse-Kassel.

Although William was of royal blood (his mother and father were both great-grandchildren of Frederick V of Denmark and great-great-grandchildren of George II of Great Britain) his family was relatively obscure and lived a comparatively normal life by royal standards. In 1852, however, his father was designated the heir presumptive to the childless Frederick VII of Denmark, and the following year the family became princes and princesses of Denmark. William's siblings were Frederick (who succeeded their father as King of Denmark), Alexandra (who married Edward VII of the United Kingdom), Dagmar (who married Alexander III of Russia), Thyra (who married Ernest Augustus, Crown Prince of Hanover) and Valdemar.

William's mother tongue was Danish, with English as a second language. He was also taught French and German. He embarked on a career in the Royal Danish Navy, and enrolled as a naval cadet along with his elder brother Frederick. While Frederick was described as "quiet and extremely well-behaved", William was "lively and full of pranks".

Following the expulsion of Otto of Greece in October 1862, the Greek people had rejected Otto's brother and designated successor Luitpold, although they still favored a monarchy rather than a republic. Many Greeks, seeking closer ties to the pre-eminent world power, the United Kingdom, rallied around Prince Alfred, Duke of Edinburgh, second son of Queen Victoria and Prince Albert. British prime minister Lord Palmerston believed that the Greeks were "panting for increase in territory", hoping for a gift of the Ionian Islands, which were then a British protectorate. The London Conference of 1832, however, prohibited any of the Great Powers' ruling families from accepting the crown. Queen Victoria herself was adamantly opposed to the idea. The Greeks nevertheless insisted on holding a plebiscite in which Alfred received over 95% of the 240,000 votes. There were 93 votes for a Republic and six for a Greek national to be chosen as king. King Otto received one vote.

With Prince Alfred's exclusion, the search began for an alternative candidate. The French favored Henri d'Orléans, duc d'Aumale, while the British proposed Queen Victoria's brother-in-law Ernest II, Duke of Saxe-Coburg and Gotha, her nephew Prince Leiningen, and Archduke Maximilian of Austria, among others. Eventually, the Greeks and Great Powers winnowed their choice to Prince William of Denmark, who had received six votes in the plebiscite. Aged only 17, he was elected King of the Hellenes on 30 March [O.S. 18 March] 1863 by the Greek National Assembly under the regnal name of George I. Paradoxically, he ascended a royal throne before his father, who became King of Denmark on 15 November the same year. There were two significant differences between George's elevation and that of his predecessor, Otto. First, he was acclaimed unanimously by the Greek Assembly, rather than imposed on the people by foreign powers. Second, he was proclaimed "King of the Hellenes" instead of "King of Greece", which had been Otto's style.