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.

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 drains into the Missouri River. This project irrigates 51,429 acres (208.13 km²) and serves water to 450 farms, according to the manager of the Lower Yellowstone Irrigation Districts.

During the Great Depression, Montana artist J. K. Ralston painted a Federal Arts Project mural at the Richland County Courthouse in Sidney.

The town received a boost in 1924 when the Holly Sugar Corporation opened up a sugar refinery in Sidney to process locally grown sugar beets.

The area experienced an oil boom and bust in the late 1970s and early 1980s, bringing an influx of people to the town for a short period of time. Around the start of the 21st century, the town experienced another surge in oil exploration activity.

Sidney is located in the northeastern part of the state.