Sidney is a city in and the county seat of Richland County, Montana, United States, less than 10 mi (16 km) west of the North Dakota border. The population was 6,346 at the 2020 census, and was estimated to be 6,031 in 2024. The city lies along the Yellowstone River. Sidney is approximately midway between Glendive, Montana and Williston, North Dakota.