Colby
- County:
- Thomas County
- County Seat:
- Yes
- Area (mi²):
- 3.661
- State:
- Kansas
Colby is a city located in Thomas County, Kansas. Colby has a 2026 population of 5,539. It is also the county seat of Thomas County. Colby is currently declining at a rate of -0.65% annually and its population has decreased by -1.25% since the most recent census, which recorded a population of 5,609 in 2020.