Yes, you will need to obtain a multiple-entry visa. Without it you may have to begin your visa process all over again. Since procedures/regulations can vary depending on your passport/country of origin, you may want to contact the immigration helpline to find out how long it would take to get your multiple-entry visa.
Here is a link to a page with contact information for you:
http://www.korea4expats.com/article-immigration-helpline-websites.html