I am a U.S. citizen. Does South Korea stamp my passport upon entry to the country? They did not stamp it when I left two weeks ago. My passport's visa pages are almost filled, but it is still good for the next six years. I want to know if I need to get a new passport for additional visa pages.
No, since November 2016 South Korea does not give any exit stamps to anyone.
I added this answer to clarify the discrepancy between the other answers and give a clear source for the information.
There was a change of practise in November 2016, when exit stamps were discontinued:
The Ministry of Justice has decided to dispense with the stamping process for departure immigration clearance starting from 1 November 2016 for all passengers (both Korean and foreign nationals) departing the Republic of Korea in order to provide a more convenient and simplified service in an era of 70 million travelers.
The aim of this move is to streamline the process:
The average departure immigration clearance time per passenger would be shortened from 18 seconds to 15 seconds for Korean nationals, and 23 seconds to 20 seconds for foreign nationals.
I have had exit stamps from Korea in the past, but as rules for Americans visiting Korea are pretty relaxed you likely will not encounter any issues (plus it seems you need to replace your passport anyway).
You can no longer add additional pages to a US Passport, so if you are out of pages you need to apply for a renewal early. Your current passport will be voided when you renew, but most visas you currently have will remain valid and you simply need to carry both passports with you when traveling to those destinations.
I have been to South Korea in Nov 2016. Got a regular Korean square entrance stamp, stating the entry date and the duration of stay (entered via PUS airport if it matters).
However there are no exit stamps anymore in Korea, and the immigration desk has a printed note in English saying "Korean Immigration will not anymore stamp your passport on exit". This was in GMP, but the immigration rules shall apply uniformly. This is a new rule, as even in 2015 I got an exit stamp
Besides this, I can also say the immigration doesn't look at your entrance stamp either (or anywhere except the photo page of your passport). Passport into the scanner, and you get it back right away. So even if there are no entry stamps, it is not going to get noticed.