Any foreigner wishing to work in Korea needs to possess a valid passport and obtain the appropriate visa from a Korean embassy or consulate before entering the country. A work-related visa cannot be issued inside Korea. Working in Korea without a proper visa is strictly prohibited and violators can be expelled from the country

A person wishing to work in Korea must apply for the appropriate visa before entering the country. Work-related visa applications can take over a month to process, so applications should be made well in advance of intended travel.

Please see http://www.moj.go.kr for a full list of visa types and required documents.