Colby is a city in and the county seat of Thomas County, Kansas, United States. As of the 2020 census, the population of the city was 5,570.

Thomas County is a county located in the U.S. state of Kansas. Its county seat is Colby. As of the 2020 census, the county population was 7,930. The county was named after George Thomas, a general during the American Civil War.