For amounts exceeding 10,000 USD, you must declare them to the customs officer upon arrival in South Korea. You will need to fill out a Currency Declaration form and present it to the customs officer along with your passport and any other relevant documents.
If you fail to declare cash in excess of 10,000 USD, you may face penalties, including confiscation of the undeclared cash and fines.
In addition to the cash declaration requirement, you are also required to declare any other valuable items, such as jewelry, electronics, and antiques.