I'm traveling to South Korea on single entry visa and I'm making plans of visiting Jeju island during my stay from Seoul. As I have my return flight back from Seoul I have to travel back to Seoul again from Jeju island. Is it possible to execute this plan with single entry visa to south Korea?
Flights between mainland Korea and Jeju island are considered domestic flights and there is no passport control. You only need your ID (obviously, passport). Since you have a valid visa for Mainland Korea, Jeju island is automatically covered by your single visa.