Otto I (Greek: Όθων, romanized: Óthon; German: Otto Friedrich Ludwig von Wittelsbach; 1 June 1815 – 26 July 1867) was King of Greece from the establishment of the Kingdom of Greece on 7 May 1832, under the Convention of London, until he was deposed in October 1862.

The second son of King Ludwig I of Bavaria, Otto ascended the newly created throne of Greece at age 17. His government was initially run by a three-man regency council made up of Bavarian court officials. Upon reaching his majority, Otto removed the regents when they proved unpopular with the people, and he ruled as an absolute monarch. Eventually, his subjects' demands for a constitution proved overwhelming, and in the face of an armed (but bloodless) insurrection, Otto granted a constitution in 1843.

Throughout his reign, Otto tried to make significant reforms to modernize Greece, seeing himself as Enlightened absolutist. He established educational institutions and several state services but was unable to resolve Greece's major part of poverty and prevent economic meddling from outside. Greek politics in this era were based on affiliations with the three Great Powers that had guaranteed Greece's independence, Britain, France and Russia, and Otto's ability to maintain the support of the powers was key to his remaining in power. To remain strong, Otto had to play the interests of each of the Great Powers' Greek adherents against the others, while not irritating the Great Powers. When Greece was blockaded by the British Royal Navy in 1850 and again in 1854, to stop Greece from attacking the Ottoman Empire during the Crimean War, Otto's standing amongst Greeks suffered. As a result, there was an assassination attempt on Queen Amalia, and finally, in October 1862, Otto was deposed while in the countryside. He died in exile in Bavaria in 1867.

Otto was born as Prince Otto Friedrich Ludwig of Bavaria at Schloss Mirabell in Salzburg (when it briefly belonged to the Kingdom of Bavaria), as the second son of Crown Prince Ludwig of Bavaria and Therese of Saxe-Hildburghausen. His father served there as the Bavarian governor-general. Through his ancestor, the Bavarian Duke John II, Otto was a descendant of the Byzantine imperial dynasties of Komnenos and Laskaris. His father was a prominent Philhellene, and provided significant financial aid to the Greek cause during the War of Independence.

Otto was a child of delicate health and temperament, with a slight stutter and a passion for the piano. As a teenager, he was tutored in Classical Greek and Latin by classical scholar and passionate Philhellene Friedrich Thiersch, who was the first to suggest the young prince as a candidate for the throne of the emerging nation. Thiersch's suggestion was supported by Jean-Gabriel Eynard, a major financial benefactor of the Greek independence movement and friend of Ioannis Kapodistrias, Greece's governor. Otto's name therefore entered the discourse surrounding Greek independence, both within Greece and abroad.

At the end of Greek War of Independence, the three Great Powers formulated the London Protocol of 1829, which recognized an autonomous Greek state. Article 3 of the protocol stated that Greece would be a monarchy, under the rule of a prince who was not from the ruling families of one of the three Great Powers. Numerous candidates were considered for the vacant throne, including Prince Frederick of the Netherlands and Otto's uncle, Prince Karl Theodor of Bavaria. Even an Irishman named Nicholas Macdonald Sarsfield Cod'd put himself forward, claiming descent from the Byzantine Palaiologos dynasty. Ultimately, they settled on Prince Leopold of Saxe-Coburg and Gotha, and the amended London Protocol of 1830 recognized him as the de jure sovereign of Greece. Although initially enthusiastic, Leopold was discouraged by the limited borders established by the protocol and Britain's refusal to grant financial support to the vulnerable new state. Due to this, as well as personal reasons, he formally rejected the crown three months later. Kapodistrias' assassination in 1831 destabilized Greece, and caused British Foreign Secretary Lord Palmerston to convene the London conference. Here, the crown was offered to the 17-year-old Prince Otto, which he happily accepted. The Bavarian House of Wittelsbach had no connections to the ruling dynasties of any of the Great Powers, and so was a neutral choice with which they were all satisfied. The Greeks were not consulted, but Greece was in chaos and no group or individual could claim to represent it anyway. The London Protocol of 1832 therefore finally recognized Greece as a fully independent state, with Otto as its king.

The Great Powers extracted a pledge from Otto's father to restrain him from hostile actions against the Ottoman Empire. They also insisted that Otto's title would be "King of Greece", rather than "King of the Hellenes", because the latter would imply a claim over the millions of Greeks then still under Turkish rule. Not quite 18, the young prince arrived in Greece with 3,500 Bavarian troops (the Bavarian Auxiliary Corps) and three Bavarian advisors aboard the British frigate HMS Madagascar. Although he did not speak Greek, he immediately endeared himself to his adopted country by adopting the Greek national costume and Hellenizing his name to "Othon" (some English sources, such as Encyclopædia Britannica, call him "Otho"). Thousands lined the docks of Nafplio to witness his arrival, including many heroes of the revolution such as Theodoros Kolokotronis and Alexandros Mavrokordatos. His arrival was initially enthusiastically welcomed by the Greek people as an end to the chaos of the prior years and the beginning of the rejuvenation of the Greek nation. A year later Greek poet Panagiotis Soutsos evoked the scene in Leander, the first novel to be published in independent Greece:

O King of Greece! Old Greece bequeathed the lights of learning to Germany, through you Germany has undertaken to repay the gift with interest, and will be grateful to you, seeing in you the one to resurrect the firstborn people of the Earth.