Johnston County is a county located in the U.S. state of Oklahoma. As of the 2020 census, the population was 10,272. Its county seat is Tishomingo. It was established at statehood on November 16, 1907, and named for Douglas H. Johnston, a governor of the Chickasaw Nation.

Tishomingo is the largest city in, and the county seat of, Johnston County, Oklahoma, United States. The population was 3,034 at the 2010 census, a decline of 4.1 percent from the figure of 3,162 in 2000. It was the first capital of the Chickasaw Nation, from 1856 until Oklahoma statehood in 1907. The city is home to Murray State College, a community college with an annual enrollment of 3,015 students. Tishomingo is part of the Texoma region.