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 5,191 at the 2010 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.
Settlers began arriving in the area in the 1870s, and a post office was established in 1888. Six-year-old Sidney Walters and his parents were staying with Hiram Otis, the local justice of the peace, and Otis decided that Sidney was a good name for the town. The following year, Montana became a state and Sidney was incorporated in 1911. 
Sidney was originally part of Dawson County, but became the county seat of Richland County at its inception in 1914.
Agriculture became an important part of the region after the Lower Yellowstone Irrigation Project was completed in 1909. A dam was built on the river south of Glendive, which diverted water from the river into a 115.2 km (71.6 mi) main canal, which runs north-south, parallel to the Yellowstone, irrigating land from Glendive north up to Fairview, where it dumps into the Missouri River.