Grant County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the population was 7,233, making it Oregon's fourth-least populous county. The county seat is Canyon City. It is named for President Ulysses S. Grant, who served as an army officer in the Oregon Territory, and at the time of the county's creation was a Union general in the American Civil War. Grant County is included in the eight-county definition of Eastern Oregon.

Grant County was established on October 14, 1864, from parts of old Wasco and old Umatilla counties. Prior to its creation, cases brought to court were tried in The Dalles, county seat of the vast Wasco County. The great distance to The Dalles made law enforcement a difficult problem, and imposed a heavy burden on citizens who had a need to transact business at the courthouse. In 1889, more than half of the southern part of the original Grant County was taken to form Harney County. Also in 1899, a small part of northwestern Grant County was taken (along with parts of Crook and Gilliam counties) to form Wheeler County.

After gold was discovered in 1862 on Whiskey Flat, it has been estimated that within ten days 1,000 miners were camped along Canyon Creek. This increased population created a need for county government. Grant County's government operates in accordance with the Oregon Constitution which was ratified by the People of Oregon in November 1857, and the revised Statutes of Oregon. It employs the old-western county government system: the County Court, with a County Judge and two Commissioners. While the County Court no longer exercises much judicial authority, it serves as the executive branch of county government.

The third man to serve as County Judge of Grant County was Cincinnatus Hiner "Joaquin" Miller (1837–1913), the noted poet, playwright, and western naturalist, called the "Poet of the Sierras" and the "Byron of the Rockies."

The county seat is Canyon City, which served as the chief community of the county for many years. In 1864, when the county was organized, Canyon City is said to have boasted the largest population of any community in Oregon. Mining and ranching, along with timber and then the service and public works that followed, brought people into the area and communities grew around the natural centers of industry and agriculture. Canyon City hosts an annual summer festival called "'62 Days" (referencing the local gold discovery in 1862) to celebrate its history and residents.

Since the 1930s, the city of John Day has served as the main economic center of the county, and boasts the largest population.

According to the United States Census Bureau, the county has a total area of 4,529 square miles (11,730 km²), of which 4,529 square miles (11,730 km²) is land and 0.7 square miles (1.8 km²) (0.02%) is water.

Approximately 63% of the land area of the county is controlled by the Federal Government, most of which is controlled by the U.S. Forest Service, and the Bureau of Land Management. Grant County contains most of the Malheur National Forest and sections of the Wallowa–Whitman, Umatilla and Ochoco National Forests, and has more than 150,000 acres (610 km²) of federally designated Wilderness Areas.