Per U.S. Embassy in Seoul website:
All travelers to Korea must provide proof of a negative COVID-19 PCR test result issued within 72 hours of their departure and submit to a mandatory 14-day quarantine upon entry, regardless of vaccination status. Most tourists must quarantine in government facilities at their own expense. COVID-19 screening measures are subject to change with little advance notice. At this time, the ROK government has not changed its testing and quarantine requirements with regard to vaccinated travelers. General information is detailed below, but please consult the ROK COVID-19 Portal, Korea Disease Control and Prevention Agency (KDCA) and the Embassy of the ROK in the USA for the most up-to-date information on special COVID regulations and immigration procedures before your travels.