Roundup is a city in and the county seat of Musselshell County, Montana, United States. The population was 1,931 during the 2000 census.
Roundup is located at 46°26′54″N 108°32′34″W / 46.44833°N 108.54278°W / 46.44833; -108.54278 (46.448401, -108.542676)
The Musselshell River runs through the city. Roundup is surrounded by the Bull Mountains which is a part of the Rocky Mountains.