Citizens of the UK, most EU countries, Australia, New Zealand and the United States can stay in South Korea as a tourist for up to 90 days without a visa. Portuguese passport holders are permitted a stay of up to 60 days and Canadian citizens can stay for up to six months. Your passport should be valid for a minimum period of six months from the date of entry into South Korea.
You will need to fill in an online form for a Korea Electronic Travel Authorization (K-ETA) prior to travel with a small fee incurred.
All other nationalities should check with the Korean Embassy in their country of residence for the most up-to-date information.