Celaya is a city and its surrounding municipality in the state of Guanajuato, Mexico, located in the southeast quadrant of the state. It is the third most populous city in the state, with a 2005 census population of 310,413. The municipality for which the city serves as municipal seat, had a population of 415,869. The city is located in the geographic center of the municipality, which has an areal extent of 553.18 km² (213.58 sq mi) and includes many smaller outlying communities, the largest of which are San Miguel Octopan, Rincón de Tamayo and San Juan de la Vega.