Hamilton, Montana. Hamilton (Salish: čɫc̓lc̓lé ) 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.