Jeju is the one place in Korea where a rental car is not optional — the buses exist but run infrequently and the island's sights are scattered. Here is everything about renting and driving there.
Requirements
You need your home license plus an International Driving Permit (IDP) — the 1949 Geneva convention version, which Korea recognizes; the 1968 Vienna version is NOT accepted. Get it before you leave home; it cannot be issued abroad. You also need your passport and a credit card. Most rental companies require drivers to be 21+ with one year of license history; some set 26+ for larger cars.
Booking and Cost
Book ahead online — Lotte Rent-a-Car and SK Rent-a-Car are the big operators with English booking and airport desks; the smaller local companies are cheaper but often Korean-only. A compact car runs roughly 50,000–80,000 KRW per day in shoulder season, more in summer. Take the full insurance option (the "super" coverage waiving the deductible) — Jeju's narrow stone-walled lanes and tight parking make scrapes genuinely common. Electric cars are popular and cheap to run; chargers are everywhere on the island.
The Driving
Koreans drive on the right. Jeju driving is relaxed by mainland standards but has quirks: the island's speed limits are low (mostly 50–80 km/h) and heavily camera-enforced — the rental GPS/nav will announce every camera, listen to it. Many rural roads are single-lane between stone walls with no shoulder. Yellow center lines mean no passing, and Jeju enforces it. Gas stations are common; many rural ones are self-service with Korean-only pumps — the card readers take foreign cards at most stations.
Navigation
Google Maps does not work properly in Korea. Use the rental car's built-in nav (most have English mode — ask them to set it at pickup) or Naver Map/KakaoMap on your phone. Entering a phone number of your destination into the Korean nav is the local trick and works better than addresses.
The Alternative
If you cannot drive: the 201/211/281 buses cover the main east-west routes, the airport limousine covers the resorts, and day-tour buses hit the highlights — but expect to see half as much per day.