Foreigner Registration

For those of you who are US citizens, when you come to Korea, in most cases, you should already have your E2, F2 or F4 visa prior to arriving. In either case, the US Embassy of Korea offers you the recommended option to register with them. People of other citizenship can also make use of this service with their respective embassies in Korea.
The US Embassy of Seoul highly suggests that you register via the US State Department website. It is by no means mandatory, but it allows them to know that you are an American citizen in Korea whether it's long- or short-term. They suggest this option in case of family emergencies, for example, so that the US Embassy can contact you should your family not be able to do so.

There are a few perks that registering with the embassy online can offer. You can update your information at any time when traveling. Therefore, if you take a trip to Japan, you can visit the website mentioned above to change the foreign country in which you are located, so the American embassy in that country can contact you in case of emergency. You will also receive monthly emailed newsletters for the American expat community in Korea as well as updates on safety and security for Americans abroad. Finally, you can also opt to receive text messages when updates have been made.

It should be noted that your data entered on the US Department of State's website will be kept private from any personnel outside of the embassies, consulates and Department of State. It will be released only with your permission. Also, if you have already registered before July 2004, it is advised to re-register.