Clayton County is located in the north central portion of the U.S. state of Georgia. As of the 2020 census, the population was 297,595 by the U.S. Census Bureau. The county seat is Jonesboro.

Clayton County is one of the core counties of the Atlanta metropolitan area, and it is the fifth most-populous county in the state. It is the home of most of Hartsfield–Jackson Atlanta International Airport, the busiest airport in the world by total passengers.

The county was the defendant in the landmark LGBT rights case of Bostock v. Clayton County (2020).

The county was established in 1858 and named in honor of Augustin Smith Clayton (1783–1839), who served in the United States House of Representatives from 1832 until 1835.

Clayton County was a battle site during the American Civil War, with the Battle of Jonesborough and the Battle of Lovejoy's Station taking place in the area.

According to the U.S. Census Bureau, the county has a total area of 144 square miles (370 km²), of which 142 square miles (370 km²) is land and 2.8 square miles (7.3 km²) (1.9%) is water. It is the third-smallest county by area in Georgia.

The eastern portion of Clayton County, between Forest Park and Lovejoy, is located in the Upper Ocmulgee River sub-basin of the Altamaha River basin. The western portion of the county is located in the Upper Flint River sub-basin of the ACF River Basin (Apalachicola-Chattahoochee-Flint River Basin).

At the 2020 United States census, there were 297,595 people, 97,030 households, and 62,746 families residing in the county. In 2000, it had a population of 236,517.