Rancho Viejo is a town in Cameron County, Texas, United States. The population was 2,838 at the 2020 census, up from 2,437 at the 2010 census. The town is named from a ranch in that area named "Rancho Viejo" (Old Ranch). This ranch was owned by Blas María de la Garza Falcón, whose name was given to a nearby dam, Falcon Dam.