For European citizens and from Switzerland (1), no visa is required for stays shorter than 90 days.
It´s always obligatory to present your passport, valid till at least 6 months after return date. Sometimes you also have to show your return flight ticket.
For more information, you can visit the website of the South Korean Embassy in Belgium
(1) Other nationalities have to find information on the website of the South Korean Embassy of their country of origin.