Once interest and dividends exceed 20 million won a year, the excess is taxed at progressive rates. But a comparative-taxation rule keeps the effective rate at 14% up to roughly 81 million won for people with no other income. Health insurance moves first.
The tax on a yen gain depends on the wrapper. FX deposit currency gains are untaxed, but a Korea-listed yen ETF's trading profit is dividend income withheld at 15.4% and counted toward the aggregate financial income threshold.
US dividends lose 15% at source and nothing more in Korea — lighter than the 15.4% on Korean dividends. The real split comes at 20 million won of annual financial income.
Only financial income above 20 million won gets rolled into comprehensive taxation, but health insurance starts counting at 10 million won. A 1-million-won difference can create 480,000 won a year in premiums.