Hamilton is a city in and the county seat of Ravalli County, Montana, United States. The population was 4,348 at the 2010 census. Significant outlying population growth is shown in the area; the ZIP Code Tabulation Area for Hamilton's ZIP Code, 59840, had a population of 12,979 at the United States Census Bureau 2013 estimate. If the city of Hamilton annexed all the area in its zipcode it would be the 8th largest city in Montana.