How do I use Korean documents such as diplomas, criminal background checks, and marriage certificates in another country?

For Korean public documents like diplomas from Korean schools, criminal background checks, and marriage certificates used abroad, you get an apostille from the Ministry of Foreign Affairs (some documents go through the Ministry of Justice). You can apply at the MOFA consular/apostille office in person, and certain documents support e-apostille online. If your destination country isn't part of the apostille convention, you instead need consular legalization at that country's embassy in Korea.