Harlan County is a county in the U.S. state of Nebraska. As of the 2020 United States Census, the population was 3,073. Its county seat is Alma. The county is home to the Harlan County Reservoir. In the Nebraska license plate system, Harlan County is represented by the prefix 51 (it had the 51st-largest number of vehicles registered in the county when the license plate system was established in 1922).