South Korea has some of the best mobile infrastructure in the world. In Seoul you will almost never lose signal, including in the deepest subway stations. But "almost" still has exceptions.
Inside Seoul
Nothing to worry about. All three Korean networks — SK Telecom, KT, LG U+ — cover the city completely, including subway lines and shopping-centre basements. Speeds are usually well above anything you need for maps and messaging.
Outside Seoul, a few places worth knowing
- Jeju Island: the coastal ring road has good coverage, but the trails up Hallasan are patchy. If you plan to hike, download offline maps first.
- The DMZ: areas near the border have restricted coverage. Tours travel as a group so it rarely matters, but do not rely on data to go off on your own.
- Historic villages such as Hahoe and Jeonju: coverage is fine, just slower than the city.
How much data
People tend to use more data in Korea than in Japan, for two reasons: they take and post more photos, and they often use Naver Map or KakaoMap instead of Google Maps — both heavier apps.
- Ordinary sightseeing, seven days: 2 GB per day is comfortable.
- Shooting video and posting constantly: 3 GB per day.
- Working remotely or sharing to a laptop: 5 GB per day or more, and check whether the plan allows hotspot sharing.
Google Maps is incomplete in Korea
This surprises most first-time visitors. Because of Korean mapping regulations, Google Maps does not have detailed walking and driving directions the way it does elsewhere. Locals use Naver Map or KakaoMap.
Both have English and work well, but download and sign in to them before you fly, while you still have home wifi. Doing it on arrival wastes time and data on your first day.
About phone numbers
A travel eSIM is data only — it does not give you a Korean phone number. That matters more than it sounds: some Korean services, including restaurant bookings and certain ride apps, want a local number for verification. If your plans depend on those, arrange a fallback, such as asking your hotel to book for you.