What are the requirements for entry to South Korea?

Entry, Exit and Visa Requirements

You must have a valid U.S. passport to enter Korea. ...
Visa required for all other purposes, including employment, teaching English, and for stays longer than 90 days.

What app do I need to enter Korea?

Please be informed that you need K-ETA approval before boarding the flight or ship bound for the Republic of Korea. You can apply for K-ETA via the K-ETA official website (www.k-eta.go.kr) or mobile App (K-ETA).