San Diego de la Unión is a city located in the Northwest region of the state of Guanajuato, Mexico. The municipality covers a total area of 990.17 square kilometres.
The region is bordered by San Luis Potosí to the north, San Luis de la Paz to the east, Dolores Hidalgo to the south, and San Felipe to the west. According to the 2005 census, the municipality had a population of 34,088 inhabitants.