St. Marie, MT has a population of 598 and is the 18,461st largest city in the United States. The population density is 26 per sq mi which is 283% higher than the Montana average and 71% lower than the national average. The median age in St. Marie is 36 which is approximately 11% lower than the Montana average of 40. In St. Marie, 78% of the population over 15 years of age are married, 96% speak English and 0% speak Spanish. 43% of St. Marie residents were born in Montana, 52% were born out of state, 0% were born outside of the United States and 5% were foreign born.