In Korea, do not assume you need to add the 10–20% restaurant tip common in some countries. Korea Tourism Organization's current traveler guidance says tipping is not customary and describes prices as including service charges.
For an ordinary restaurant meal, cafe purchase or taxi ride, that means you generally pay the displayed or metered amount rather than calculating an extra percentage. Do not pressure yourself to leave cash on the table simply because you would do so at home.
Still, treat this as a national custom rather than a promise about every private business. A hotel, private tour, luxury venue, international brand or booking platform can set its own service or gratuity policy. If a bill or booking page explicitly states a service charge, package gratuity or optional tip, read that specific policy rather than relying on a general rule.
A related etiquette point from KTO is that shoes should be removed when entering homes or restaurants with ondol-style floor rooms. Follow the visual cue at the entrance or the host's instruction rather than removing shoes in every restaurant.
Practical next step: in a normal restaurant or taxi, pay the stated amount unless the business clearly tells you otherwise. For a private tour or premium service, check the booking terms before arrival.