San Diego de la Unión is a Mexican city located in the Northwest region of the state of Guanajuato. The municipality has an area of 990.17 square kilometres and is bordered to the north by San Luis Potosí, to the east by San Luis de la Paz, to the south by Dolores Hidalgo, and to the west by San Felipe. The municipality had 34,088 inhabitants according to the 2005 census. The municipality of San Diego was founded in 1719 under the name of Pueblo de Bizcocho.