Grant County is a county in the U.S. state of Nebraska. As of the 2020 United States Census, the population was 611, making it the fourth-least populous county in Nebraska and the ninth-least populous county in the United States. Its county seat is Hyannis. The county was founded in 1887 and named for Ulysses S. Grant, 18th President of the United States. In the Nebraska license plate system, Grant County is represented by the prefix 92.