San Luis Río Colorado is a city and also the name of its surrounding municipality in the state of Sonora, Mexico. In the 2020 census, the city had a population of 176,685. The city is the fourth-largest community in the state, and the municipality is also the fourth-largest in terms of population. Lying in the northwestern corner of Sonora, the city marks the state border with Baja California. It also stands on the international border with the United States, adjacent to San Luis, Arizona. It is located about 75 km from Mexicali. The municipality covers an area of 8,412.75 km² (3,248.2 sq mi) in the Sonoran Desert.