Missoula County is located in the State of Montana. As of the 2020 census, the population was 117,922, making it Montana's third most populous county. Its county seat and most populous city is Missoula. The county was founded in 1860.

Missoula is a city in and the county seat of Missoula County, Montana, United States. It is located along the Clark Fork River near its confluence with the Bitterroot and Blackfoot Rivers in western Montana and at the convergence of five mountain ranges, thus it is often described as the "hub of five valleys". The 2020 United States Census shows the city's population at 73,489 and the population of the Missoula Metropolitan Area at 122,458. Missoula is the 2nd largest city and metropolitan area in Montana. Missoula is home to the University of Montana, a public research university.