Based on my 5-day family trip, Korea is mostly cashless unless you do not buy the tourist pass in advance at the airport to cover transit tickets (subway/bus). If you're traveling as an ApplePay or credit card carrying adult it's mostly cashless. KRW 30,000 (under US$25) is enough for a week.

Seoul

Use Apple's T-Money card which is now available for use by tourists (as of April 2026). It's a terrible user experience because you still need to download an app instead of just adding and funding via Apple Wallet, but it does work.

Some restaurants are cash-only still, but you can easily skip those.

Toilets occasionally charge also.

There is no tipping in Korea. I have even seen an American tourist get chased by a taxi driver outside the Grand Hyatt for tipping. DO NOT TIP. It is insulting to Koreans.

Nami Island

Cash for some snacks is recommended.

Suwon

Some coffee shops are cash-only.