Diogo das Chagas (c.1584;Santa Cruz das Flores — c.1661; Angra do Heroísmo) was a Franciscan monk and Azorean historian, author of Espelho Cristalino, an important resource on the colonization of the islands of the central and western groups of the Portuguese Azores, after 1640.
He was the son of Mateus Coelho da Costa, Captain-major of the island of Flores, and his wife, D. Catarina de Fraga Rodovalho.
Little is known of his infancy and childhood; Chagas wrote that his first studies occurred in the city of Angra, where he received ecclesiastical training, but, owing for the absence of a bishop in the Diocese de Angra, he travelled to Lisbon in 1612, in order to be ordained priest. He returned in 1614, and began studying Art at the Colégio da Companhia de Jesus in Angra. In 1616, he registered in Theology at the University of Coimbra. He completed his studies in 1620, and returned to the Azores and taught Theology at the Convent of São Francisco in Angra.
He became guardian of the Convent of São Francisco in Praia (in 1627) and church orator (in 1629). Along with his brother, Friar Mateus da Conceição, he was responsible for convincing the Franciscan hierarchy to de-annex the Azores from the Província Franciscana dos Algarves (English: Franciscan Province of the Algarves), to a wholly local Província Franciscana de São João Evangelista dos Açores (English: Franciscan Province of Saint John the Baptist of the Azores) administration, in 1638. It was only after the aclamation of John IV of Portugal that the new entity was established, in 1641.
A friend of Captain-major Francisco de Ornelas da Câmara, Chagas was a vocal supporter of the Guerra do Castelo (English: Battle of the Castle) during the Portuguese Restoration War, (27 March 1641 - 4 March 1642), which was responsible for the surrender of the Spanish garrison at the Fort of São João Baptista, in Terceira.
In 1646, he was made provincial vicar of the Franciscan Order in the Azores, a position he would hold until 1649. During his time in the role, he traveled successively throughout the archipelago, in order to meet with the local people and explore his faith community. He was considered the most dignified Father in 1655, at the time that new provincial elections were held.