Koldo Mitxelena Elissalt (Basque pronunciation: [koldo mitʃelena]) (also known as Luis Michelena; 1915, Errenteria, Gipuzkoa – 11 October 1987, San Sebastián) was an eminent Basque linguist. He taught in the Department of Philology at the University of the Basque Country, and was a member of the Royal Academy of the Basque Language.
He is described as "the greatest scholar the Basque language has ever seen." He is known for the complete reconstruction of Proto-Basque he undertook in the 1950s, as well as the formal demonstration in 1954 that the Aquitanian language was an ancestral form of Basque. Mitxelena was also one of the main participants in the creation of "Euskara Batua" or Standard Basque. In 1987 he was declared Seme Kuttun of the City of Errenteria, literally 'beloved son'.
Mitxelena was born into a family engaged in industrial crafts. When still a child, illness kept him bed-ridden for a long time. This awoke his interest in reading, and also his love for the Basque language.
Mitxelena came from a Basque Nationalist family. When he was young, he was affiliated with the Euskal Langileen Alkartasuna and the Basque Nationalist Party, and learnt about the Euskaltzaleak youth movement led by Jose Ariztimuño, known as "Aitzol." He began working in a factory while continuing his studies, also starting to attend meetings. At this time he became interested in pre-war Basque literature.
When Francisco Franco's military coup took place, Mitxelena volunteered to join the Euzko Gudarostea nationalist Basque army. He was taken captive in Santoña and was condemned to death on September 7, 1937, but the sentence was commuted to 30 years in prison. He thus came to know the prisons of El Dueso, Larrinaga, and Burgos.