Grady County is a county located in the U.S. state of Oklahoma. As of the 2020 census, the population was 54,795. Its county seat is Chickasha. It was named for Henry W. Grady, an editor of the Atlanta Constitution and southern orator.

Grady County is part of the Oklahoma City metropolitan area.

Grady County was part of the land given to the Choctaw by the Treaty of Dancing Rabbit Creek, in exchange for property in the southeastern United States. In 1837, the Chickasaw joined the Choctaws, and in 1855 a treaty separated the two tribes, and the Chickasaw acquired an area that included much of Grady County. Most of the present Grady County became a part of Pickens County in the Chickasaw Nation.

Before the Civil War, Randolph B. Marcy blazed the California Road through this area, reporting a Waco and a Wichita village. In 1858, while the Comanches were holding a meeting with the Wichita, Choctaw, and Chickasaw, Federal troops attacked a party of Comanches. Although the commander of Fort Arbuckle had been informed about the meeting, the troops' commander, Major Earl Van Dorn, had not consulted him before the attack. As a result, the troops killed 60 Comanches and four Wichitas. Fearing a Comanche reprisal, the other tribes fled to safety at Fort Arbuckle. At the end of the Civil War, the Five Civilized Tribes and the Caddo, Delaware, Kiowa, Comanche, Apache, Cheyenne, Arapaho, and Osage signed a peace agreement and pledged to stand united against any unjust demands that the federal government made at the war's end. The agreement was known as the Camp Napoleon Compact.

The first railroad in this area was built to the town of Minco in 1890 by the Chicago, Kansas and Nebraska Railway. The company was acquired by the Chicago, Rock Island and Pacific Railroad (Rock Island) during the following year. In 1892, the Rock Island built a track connecting Chickasha, Ninnekah, and Rush Springs to the Texas border. The same railroad built a line from Chickasha to Mangum in 1900. The Oklahoma City and Western Railroad (sold to the St. Louis and San Francisco Railway in 1907) constructed tracks from Oklahoma City to Chickasha, which it extended to the Texas border in the following year. Between 1906 and 1910, the Oklahoma Central Railway (sold to the Atchison, Topeka and Santa Fe Railway in 1914) built from Lehigh to Chickasha.

The 1898 Curtis Act stripped the Chickasaw Nation of its authority, and communal land was forced into allotment, paving the way for statehood. When Oklahoma acquired statehood in 1907, the Chickasaw Nation ceased to exist, Grady County was organized and Chickasha was named the county seat. In 1911, Grady County annexed Washington, Prairie Valley, and the northern section of Dutton townships formerly in Caddo County, Oklahoma.

According to the U.S. Census Bureau, the county has a total area of 1,105 square miles (2,860 km²), of which 1,100 square miles (2,800 km²) is land and 4.4 square miles (11 km²) (0.4%) is water. The county lies in the Red Bed Plains, and is mostly covered with rolling prairie. The Canadian River forms the northern boundary and the Washita River runs through the middle.

As of the 2020 United States census, the county had a population of 54,795. Of the residents, 24.0% were under the age of 18 and 16.9% were 65 years of age or older; the median age was 39.6 years. For every 100 females there were 99.7 males, and for every 100 females age 18 and over there were 97.0 males.