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. The city lies along the Yellowstone River and is in proximity to the badlands of the Dakotas. Sidney is approximately midway between Glendive, Montana and Williston, North Dakota.

Richland County is a county in the U.S. state of Montana. As of the 2020 census, the population was 11,491. Its county seat is Sidney.