The United States of the Ionian Islands (Greek: Ἡνωμένον Κράτος τῶν Ἰονίων Νήσων, Inoménon-Krátos ton Ioníon Níson, literally "United State of the Ionian Islands"; Italian: Stati Uniti delle Isole Ionie) was a state and amical protectorate of the United Kingdom between 1815 and 1864. It was the successor state of the Septinsular Republic. It covered the territory of the Ionian Islands, in modern Greece, and it was ceded to Greece as a gift of the United Kingdom to the newly enthroned King George I at the end of the protectorate.
Before the French Revolutionary Wars, the Ionian Islands had been part of the Republic of Venice. When the 1797 Treaty of Campo Formio dissolved the Republic of Venice, it was annexed into the French Republic, created into the French departments of Greece. Between 1798 and 1799, the French were driven out by a joint Russo-Ottoman force. The occupying forces founded the Septinsular Republic, which enjoyed relative independence under nominal Ottoman suzerainty and Russian control from 1800 until 1807.
The Ionian Islands were then occupied by the French after the treaty of Tilsit. In 1809, the United Kingdom defeated the French fleet off Zakynthos island on 2 October, and captured Kefalonia, Kythira, and Zakynthos. The British took Lefkada in 1810. The island of Corfu remained occupied by the French until 1814.
The Congress of Vienna agreed to place the Ionian Islands under the exclusive "amical protection" of the United Kingdom. Despite British military administration, the Austrian Empire was guaranteed commercial status equal to the UK. The arrangement was formalised with the ratification of the "Maitland constitution" on 26 August 1817, which created a federation of the seven islands, with Lieutenant-General Sir Thomas Maitland its first "Lord High Commissioner of the Ionian Islands".
On 29 March 1864, representatives of the United Kingdom, Greece, France, and Russia signed the Treaty of London, pledging the transfer of sovereignty to Greece upon ratification; this was meant to bolster the reign of the newly installed King George I of the Hellenes. Thus, on 28 May, by proclamation of the Lord High Commissioner, the Ionian Islands were united with Greece.
According to the second constitution of the republic (1803), Greek was the primary official language, in contrast to the situation in the Septinsular Republic. Italian was still in use, though, mainly for official purposes since the Venetian Republic. The only island in which Italian (Venetian) had a wider spread was Cephalonia, where a great number of people had adopted Venetian Italian as their first language.