For a long layover or a very early flight. Cimer, the spa at the Paradise City resort, is about 1.5 km from Terminal 1, and a few more bathhouses are in the Unseo-dong hotel area towards the mainland. Check opening hours and how you will get back to the terminal before you leave the airport.
Most places on this list are ordinary neighbourhood bathhouses with signs in Korean only. These guides explain what to do at the counter, what it costs and the rules on tattoos.