Alamosa is the home rule city that is the county seat of, and the most populous municipality in, Alamosa County, Colorado United States. Alamosa is located along the Rio Grande. The city population was 9,806 in the 2020 United States census. The city is the commercial center of the San Luis Valley in south-central Colorado, and is the home of Adams State University. Alamosa is the primary city of the Alamosa, CO Micropolitan Statistical Area.