Musselshell County is a county in south-central Montana. As of the 2020 census, the population was 4,730. Its county seat is Roundup.

The Musselshell River is a tributary of the Missouri River, 341.9 miles (550.2 km) long from its origins at the confluence of its North and South Forks near Martinsdale, Montana to its mouth on the Missouri River. It is located east of the Continental divide entirely within Montana in the United States. Counting its pre-confluence tributaries, it measures 425–500 miles (684–805 km) in length.