Gaspar Castaño de Sosa (ca. 1550 - Molucca Islands, 1593) was a Portuguese conquistador, reputed slaver, and explorer who attempted to establish a colony in New Mexico in 1590.
Castaño de Sosa was born around 1550 in Portugal. He is believed by many authorities to have been a converso or "Crypto-Jew" – an ostensible Christian who continued to practice Judaism. Castaño appears in the history of northern Mexico about 1579 when along with Luis de Carabajal y Cueva he was one of the early settlers in what became the Mexican state of Nuevo León. Carbajal was governor of the province and Castaño became lieutenant governor. The two men and their group of more than sixty soldiers appear to have supported their colony by enslaving Native Americans (Indians). They raided north along the Rio Grande, capturing hundreds of Indians they sold into slavery.
In 1589, Carabajal was arrested for heresy and "Judaizing".
Castaño, fearing arrest, apparently hoped to recoup his fortunes by establishing a colony in New Mexico, which would also put additional distance between himself and the authorities in Mexico. Unable to obtain official permission for the expedition, he departed without permission from Almaden (now Monclova, Coahuila) on July 27, 1590. Thus, his journey had characteristics of both a flight from prosecution and an exploration. Accompanying Castaño were the 170 Spanish inhabitants of the town, presumably including most or all of the alleged slavers. The prospective settlers took with them a large number of livestock and carried their possessions in a slow-moving wagon train. Unlike most expeditions, no Catholic priests accompanied Castaño.