I Missed the Filing Deadline for Korean Capital Gains Tax. Here's What I Learned Doing It Myself
Most people in Korea who buy US stocks never think about capital gains tax. Not because it doesn’t exist — it does, and it’s flatter and heavier than many foreign investors expect — but because the broker handles it. You opt in during a window in spring, they pull your trades, and it’s done.
I missed that window. Then I missed the deadline behind it.
What Korea actually charges
If you’re a Korean tax resident and you sell foreign stocks at a profit, the gain is taxed at 22% — 20% national plus a 2% local surtax. One rate, no brackets.
| Item | Rule |
|---|---|
| Tax rate | 22% flat (20% national + 2% local surtax) |
| Basic exemption | ₩2.5M per year, does not carry over |
| Filing period | May 1–31 of the following year |
| Loss offset | Same calendar year, foreign stocks only |
| FX conversion | Rate on the transaction date |
| Late filing penalty | 20% of tax owed |
| Late payment penalty | 0.022% per day (~8% annually) |
| Voluntary late filing relief | 50% off within 1 month, 30% within 3 months |
There is no long-term holding discount. Eleven years and eleven days are taxed identically. Coming from a system where long-term gains get their own treatment, this is usually where people stop and re-read.
You get a basic exemption of ₩2.5 million per year, roughly $1,800. Gains under that line aren’t taxed. It’s per calendar year and it does not roll over — unused room disappears on December 31.
Losses offset gains, but only within the same year, and only against other foreign stocks. A loss on a US position cannot be set against a gain on a Korean one.
Everything is converted at the exchange rate on the transaction date, not the rate when you file. Two identical trades in dollar terms can produce different Korean tax bills.
Filing is due between May 1 and May 31 of the following year.
Why almost nobody deals with this
Korean brokers file on your behalf, usually free. It works well enough that the process underneath stays invisible. I had used it before without paying attention, and assumed I would again.
The window closed while I wasn’t looking.
Doing it by hand
Here is what the broker had been absorbing.
Every trade, one line at a time. The filing takes a spreadsheet in a fixed format — acquisition date, acquisition price, disposal date, disposal price, quantity, per lot. Not a summary, not a statement upload. Each row, typed to match the template.
You compute the tax yourself. Nothing calculates it for you. You work out the gain, subtract the exemption, apply the rate, enter the result. If your arithmetic is wrong, the form takes it anyway.
You compute the penalty yourself as well. Late filing carries a 20% penalty on the tax owed, plus 0.022% per day for late payment — around 8% a year. Both are yours to calculate and pay. This is the part I had trouble accepting. I was used to the idea that the tax office tells you what you owe. Here, being late meant working out how much extra I owed for being late, and then paying that too.
And a mistake doesn’t get fixed immediately. I got a figure wrong. I noticed shortly after submitting, went to amend — and found amendment isn’t available right after filing. The wrong number sat there, known to be wrong, while I waited.
One thing I only found afterward: filing late voluntarily reduces the 20% penalty. Within one month it’s cut in half; within three months, by 30%. Nobody tells you this at the moment you realize you’ve missed the date, and it’s the single most useful thing to know at that moment.
What I took from it
Don’t miss the broker’s window. That’s the practical lesson, and it’s not interesting, but it’s the true one.
The more useful thing was seeing the rules from the inside. ₩2.5 million isn’t a number in a table — it’s an annual allowance that expires, which makes it something to plan around rather than discover in May. The same-year rule on losses is why people here close out losing positions in December: a loss only counts against a gain in the same calendar year, so December is the last chance to use it.
I haven’t done that myself. But I understand now why people do.
Both of those rules — the expiring exemption and the same-year offset — turn out to be most of what separates direct US holdings from a Korea-listed ETF tracking the same index, which gets neither.
Figures are as of 2026 and based on National Tax Service guidance. Korean tax rules change, sometimes yearly. This is my own experience as an individual investor, not tax advice.