Diego Ortiz (c.1510–1570) was a Spanish composer and musicologist, in service to the Spanish viceroy in Naples (Pedro de Urries) and later to Philip II of Spain. Author of a treatise – Trattado de Glosas, published in 1553 – on instrumental variation technique as well as on vocal polyphonic works (magnificats and motets). Very little is known about his life. He is believed to have been born in Toledo and to have died in Naples. In 1553 Ortiz was living in the viceroyalty of Naples.