Sidney is a city in and the county seat of Cheyenne County, Nebraska, United States. The city is nine miles (14 km) north of the Colorado state line. The population was 6,410 at the 2020 census.

The city was named for Sidney Dillon, president of the Union Pacific Railroad. It was founded in 1867 by the Union Pacific and grew up around the military base of Fort Sidney (also known as Sidney Barracks), where soldiers were stationed to guard the transcontinental railroad against potential Native American attacks.

The town became the southern terminus of the Sidney Black Hills Stage Road which used Clarke's Bridge (near Bridgeport, Nebraska) to allow military and civilian traffic to reach Fort Robinson, Red Cloud Agency, Spotted Tail Agency, Custer, South Dakota, and Deadwood, South Dakota in the late 1870s and 1880s.

When the railroad reached Sidney, it was the end of a sub-division of the rail line and played host to a roundhouse, repair facilities, and a railroad hotel for passengers.

Sidney is home to one of the Old West's Boot Hill cemeteries; many of those interred there were soldiers from the fort.

The former headquarters for Cabela's was located in Sidney until 2017, when Cabela’s was acquired by Bass Pro Shops. Approximately 2,000 jobs were eliminated, while about 120 were relocated to the Bass Pro Shops headquarters in Springfield, Missouri.

According to the United States Census Bureau, the city has a total area of 6.93 square miles (17.95 km²), all land.

Sidney is toward the western edge of the Midwestern wheat-growing region. West of the city, the land is increasingly used for cattle ranching. Sidney is located along Lodgepole Creek, which is along present-day Interstate 80.