For ordinary (Class A) wage income paid by a Korean employer, the company withholds tax each month on your behalf. But if you are paid directly by a foreign employer located abroad with no Korean branch (Class B wage income), that payer is outside Korean tax law's reach, so no withholding happens automatically. Pay from a foreign company's Korean branch or office is still Class A, withheld by that branch.
Class B earners must handle their own filing and payment. Since calculating this monthly is cumbersome, most join a "Taxpayer Association," which calculates the withholding-equivalent tax each month and pays it to the tax office by the 10th of the following month on the member's behalf.
When your pay counts as Class B wage income
Why joining a taxpayer association helps
Filing monthly through a taxpayer association earns a 5% tax credit on the computed tax, and spreads the burden into small monthly payments instead of one large lump-sum filing in May. If you never joined one, you must file everything yourself during the May comprehensive income tax season.