I saw a news and check the embassy about that and sounds true Lol..

• Is a negative COVID-19 test (PCR and/or serology) required for entry? Yes, as of January 8, 2021, foreign passengers arriving in the ROK by plane must provide proof of a negative COVID-19 PCR test result obtained within 72 hours of their departure.  Airlines will not board passengers without this negative PCR test result.  Arriving passengers must submit the PCR test result on an original form issued by the testing center in the country of origin (in the English or Korean language) to quarantine authorities.  Authorities will deny entry to individuals who do not fulfill this requirement.  Policies regarding testing upon arrival and quarantine are unchanged by this announcement.  ROK authorities will also apply the same requirement to all foreign crewmen on vessels arriving at ROK seaports as of January 15, 2021.