Before you start

A view on your residence position — settled life in Korea, or 183 days
An Alien Registration Card; its thirteen-digit number is your tax identifier
Clarity on income types: Korean employment, overtime and shift premiums, foreign-source income
Resident registration at your rented address, if you want the rent credit

Step-by-step

1
Establish residence and how long you have held it
Residence turns on domicile or 183 days of presence in the tax year. Then count how many of the last ten years you have been resident: at five or fewer, foreign-source income is taxed only if a Korean entity pays it or you bring it into Korea. Crossing that threshold moves you to worldwide taxation and nothing announces it.
OnlineWho: You, with a tax accountant if you hold foreign income or assetsAssess on arrival, re-check annually
2
Model the flat 19% election properly, because here it is a real question
Foreign employees may elect a flat 19% on Korean employment income, roughly 20.9% with the local surtax, within a long window from your first working day. It replaces the progressive scale and forfeits every deduction, exemption and credit — including the rent credit and dependant allowances — and excludes anyone with a controlling interest in the employer. On the higher industrial packages Ulsan pays, the break-even is genuinely in play, and overtime-heavy years can push you across it. Run both numbers each year rather than deciding once and forgetting.
Via employerWho: You, with your employer or a tax accountantElected annually
3
Let the year-end settlement do it if you are a plain employee
Employers withhold monthly and run the year-end settlement in January and February against the tax office's pre-filled data, filing the final receipt by 10 March. Over-withholding comes back through the February or March payslip. The large Ulsan employers run this well; small subcontractors sometimes do not, so check that yours has actually filed for you.
Via employerWho: Your employer's payroll team, with documents from youJanuary–February
4
Claim the monthly rent tax credit if you are on the progressive scale
Employees below an income threshold can claim a credit on rent paid, at a rate and up to an annual cap that are both revised most years. Your resident registration must be at the rented address and the property must be within the size or value limits; landlord consent is not required. Two Ulsan-specific notes: the flat-rate election cancels this entirely, and if you live in employer-provided housing rather than renting there is no rent to credit.
Via employerWho: You, through your employer's year-end settlement or on HometaxJanuary–February
5
Know how employer-provided housing is treated
Accommodation provided by an employer is a benefit and Korea's treatment of it depends on the arrangement — who holds the lease, whether a cash allowance is paid instead, and what the contract says. That treatment is what decides whether it is taxable to you and at what value, and it is exactly the sort of question a payroll department can answer in a sentence and a forum cannot. Ask HR and, if the sums are large, a tax accountant.
Via employerWho: You with your employer's payroll and a tax accountantBefore your first settlement
6
File the May global income return if you are not a single-salary employee
Freelancers, the self-employed, people with several employers and anyone with extra reportable income file between 1 and 31 May for the previous calendar year and pay by month end. Korean clients withhold 3.3% from freelance fees as a prepayment, not a final tax. Filing on Hometax is free and the tax office runs an English helpline on 126.
OnlineWho: You, or a licensed tax accountant1–31 May

Documents you’ll need

Alien Registration Card — the registration number is your tax identifier
Withholding receipts and the year-end settlement statement from your employer
Lease, rent transfer records and resident registration abstract, for the rent credit
Deduction evidence: pension and health contributions, medical, education, dependants
Documentation of any employer-provided housing arrangement

Things most newcomers don’t know

Ulsan is the Korean city where the foreign flat-rate election most often actually wins.

The election beats the progressive scale only at higher incomes, because it forfeits every deduction and credit in exchange for a single 19% rate. The region has the highest gross regional product per head in the country and pays industrial packages to match, so a technician or engineer here can plausibly sit above the break-even where a colleague on the same job title in Gwangju does not. It is elected annually, so an overtime-heavy year can flip the answer — model it each January rather than assuming last year's conclusion holds.

Source: PwC Worldwide Tax Summaries — Korea, taxes on personal income

Electing the flat rate cancels the rent credit and every dependant allowance with it.

People model the flat rate against gross salary alone and forget what it removes. If you support dependants in Korea, pay significant medical or education costs, or rent privately and qualify for the monthly rent credit, those all disappear the moment you elect. The comparison that matters is your actual settled liability under each method, not the headline rates.

Source: National Tax Service; PwC Worldwide Tax Summaries — Korea

The tax office is also where you check a landlord's arrears before paying a deposit.

Since April 2023 a tenant whose deposit exceeds ₩10 million may inspect the landlord's unpaid national taxes at any tax office in the country, without the landlord's consent, from signing until the lease start date, with only ID and a copy of the contract. Unpaid tax outranks your deposit at auction. It is one visit to a building you will use anyway.

Source: National Tax Collection Act, as amended April 2023

The five-year window on foreign income is the most valuable relief most newcomers never hear about.

Resident five years or fewer of the last ten and your overseas salary, investments and rental income sit outside Korean tax unless a Korean entity pays them or you remit them here. That is substantial planning space for anyone with assets abroad, and it closes silently. Knowing the exact date it closes is worth an accountant's hour in year four rather than a surprise in year six.

Source: PwC Worldwide Tax Summaries — Korea, residence and income determination

Common mistakes to avoid

Never modelling the flat 19% election despite an income where it plausibly wins.
Electing the flat rate and losing the rent credit, dependant and medical deductions without counting them.
Assuming a small subcontractor employer has actually filed your year-end settlement.
Not asking how employer-provided housing is treated for tax before the first settlement.
US citizens forgetting the treaty does not remove their annual IRS filing obligation.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.