Filing in Both Korea and the US in the Same Year

People filing on both sides usually discover the problem is sequencing, not arithmetic. To claim a credit in one country for tax paid to the other, you generally need to know that other figure first — but the two filing seasons do not align, and Korea's runs after the ordinary US deadline. Planning the order, and using extensions deliberately, avoids amending a return you filed too early.

Who this affects

Korean residents with US-source income such as wages, dividends, or rental property
US persons living in Korea with Korean employment or business income
People who moved between the two countries during the tax year
Anyone claiming a foreign tax credit on either side

(001)

The calendars do not line up

Korea's 종합소득세 return is filed in May for the previous calendar year. The ordinary US individual deadline falls in April, before Korea's season has closed. US persons living abroad get an automatic extension to mid-June, which helps but still may not be enough.

(002)

Extensions are a planning tool, not an admission of lateness

Where the credit depends on a Korean figure that will not exist until after the US deadline, requesting a further US extension is ordinary practice. It is usually cheaper than filing on an estimate and amending later.

(003)

Both countries use the calendar year

One genuine simplification: Korea and the US both run the personal tax year from January to December, so income periods map cleanly onto each other. Countries with April-to-April years create far messier apportionment problems than this pair does.

(004)

Withholding is not the same as final settlement

Korean 연말정산 for employees settles Korean tax but produces no US filing, and US withholding on your W-2 is not a Korean credit until properly evidenced. Each side needs documentation the other will accept.

Common questions

Which country should I file first?

It depends on which return claims the credit. The return claiming relief for foreign tax generally needs the other country's figure first, so that one is filed second — often on extension.

Do I really have to file in the US if I live in Korea and pay Korean tax?

US citizens and green card holders file on worldwide income regardless of residence. Filing is required even in years when relief mechanisms reduce the tax owed to zero.

Does an extension to file also extend the time to pay?

No. In the US these are separate — an extension to file is not an extension to pay, and interest can accrue on tax paid after the original deadline. Estimating and paying on time while extending the filing is the usual approach.

Official sources

General information, not tax advice. Rules and thresholds change, and how they apply depends on your circumstances — confirm with a licensed professional before acting.