A Korean resident who has paid foreign income tax on foreign-source income may credit it up to a limit: Korean global income tax × (foreign-source income ÷ total global income) (Income Tax Act, Art. 57(1)). It is not a full refund but a cap at the Korean-rate equivalent.
Reverse the order and the answer is wrong
The most frequent error we see is working through the steps backwards. Starting from "I already paid in the U.S., so surely I owe nothing in Korea" almost always reaches the wrong conclusion. The order runs the other way. First, establish which country you are resident of. Second, that determines the scope of income to be aggregated in Korea. Third, if foreign-source income falls within that scope, the foreign tax credit relieves the double tax. Fourth, check whether the treaty changes any of those steps. Computing a credit before residency is settled is like computing tax before the taxable base exists.
183 days is half of the test
Article 1-2(1) defines a resident as "an individual having a domicile in Korea or a place of residence in Korea for 183 days or more". There are two conditions, joined by "or". So a person with a Korean domicile is a resident even without exceeding 183 days. Domicile is judged by where life is centred rather than by day count, so short presence can still support residence where the family lives in Korea and occupation and principal assets are there. Holding a U.S. green card or citizenship does not by itself decide Korean residency; conversely, keeping Korean nationality does not prevent non-residence where life is centred in the United States. Dual residence — both countries treating the same person as their resident — genuinely arises often. The tie-breaker rules in the Korea–U.S. treaty then assign residence to one side, and claiming treaty treatment requires supporting documents such as a tax residency certificate.
Residency widens what gets aggregated
Once you are a Korean resident, foreign-source income joins Korean income in global income. The proviso to Article 3(1) of the Income Tax Act is the exception: a foreign national resident whose periods of domicile or residence in Korea total five years or less in the ten years before the end of the tax year is taxed on foreign-source income only to the extent it is paid in Korea or remitted to Korea. It applies to foreign nationals only, not to Korean nationals returning from abroad, so nationality, the last ten years of Korean residence and whether foreign income was paid in or remitted to Korea must be checked together. U.S. salary, dividends, interest, pensions and rental income can all enter. The progressive rates in Article 55(1) then apply, rising from 6% on a base up to KRW 14m to 45% above KRW 1bn. For a non-resident the structure differs: under Article 121(2) and (3), income is aggregated where there is a domestic place of business or domestic-source real-estate income, and taxed separately by category where there is no domestic place of business. So the first question in a non-resident case is whether you file or the payer’s withholding ends it, while in a resident case it is whether all foreign income has been captured. The questions are not the same.
What the credit limit formula actually means
Treating the foreign tax credit as "Korea deducts whatever I paid in the U.S." does not reconcile. Article 57(1) states the limit as a formula: Korean global income tax multiplied by the share that foreign-source income bears to total global income. Its meaning is precise — the credit is capped at the Korean tax that would have fallen on that foreign income. Where the U.S. effective rate is higher than Korea’s, the excess sits outside the limit; where it is lower, the difference is paid in Korea. What causes trouble in practice is substantiation and conversion: which taxes count as "foreign income tax", how U.S. state income tax is treated, and which date’s exchange rate applies all move both the limit and the credit. Without the U.S. federal return and payment evidence, the credit itself becomes difficult. So we begin by setting the U.S. and Korean returns side by side and mapping the income items one to one.
The first year back — obligations start all at once
The year you wind up in the United States and return to Korea is the most complicated. Where status changes mid-year the year is split and each part taxed on its own basis, so records supporting the date of arrival and the date life shifted must be accurate. At the same time, new duties attach from the moment Korean residency begins. Article 53(1) of the Act on Adjustment of International Taxes requires a resident or domestic corporation holding overseas financial accounts to report between 1 and 30 June of the following year where the aggregate balance exceeded the threshold set by Presidential Decree on any month-end during the year. Accounts left in the U.S. can trigger this. Meanwhile U.S. filing duties generally continue for as long as citizenship or permanent residence is retained, creating a period in which both sets run together. The precise scope of the U.S. obligations needs confirmation from a U.S. tax professional. What we do is map which Korean duties begin in which year and split the income periods for that first filing year.