If we have a US Visa, is it OK to enter Seoul, South Korea to take a layover tour? It's because we will be arriving in Seoul dawn of February 21 & leave Seoul to San Francisco on the night of February 21.
It depends on your nationality. While I can't guess your nationality, if you are not part of the 24 excluded countries, you should be able to enter without visa. If you are part of the 24 countries, you're out of luck; you will need a visa.
Tourists in transit to Third Countries
1. Eligible Applicants
Nationals of countries, except for the 24 excluded countries (Syria, Sudan, Iran, Macedonia, Cuba, Republic of Kosovo, Palestine, Afghanistan, Iraq, Nigeria, Ghana, Yemen, Gambia, Senegal, Bangladesh, Kyrgyzstan, Pakistan, Somalia, Uzbekistan, Nepal, Cameron, Sri Lanka, Myanmar, Egypt) that are not granted visa free entry into Korea.
(a) Persons who hold a visa to the United States(including Guam and Saipan), Canada, Australia, or New Zealand, who transit through Korea to enter the abovementioned countries; or (b) persons who stayed in and departed from one of the abovementioned countries and transit through Korea bound for the country of origin or a third country.
(a) Persons who hold a visa (or permanent residency) to at least one of the 30 European countires (Greece, Netherlands, Norway, Denmark, Germany, Latvia, Rumania, Luxemburg, Lithuania, Liechtenstein, Malta, Belgium, Sweden, Switzerland, Spain, Slovakia, Slovenia, Iceland, Ireland, Estonia, United Kingdom, Austria, Italy, Czech, Cyprus, Portugal, Poland, France, Finland, Hungary), who transit through Korea bound for one of the 30 European countries; or (b) persons who departed from one of the 30 European countries, transit through Korea bound for the country of origin or a third country.
Persons who hold a visa (re-entry permit, permanent residency, etc.) to enter the United States, Canada, Australia, or New Zealand (hereinafter referred to as “the four advanced countries”) and
1) who go to one of the four advanced countries in transit through Korea; or
2) who stay in one of the four advanced countries and take a direct flight from the country to Korea to go to a country of nationality or a third country.
Please be noted that:
If you are a U.S. visa holder who transit through Korea and Saipan to reach the U.S. as the final destination, you are permitted to enter Korea without a visa. (However, if you depart from the U.S. and enter Korea via Saipan, you are not granted no-visa entry to Korea.)
If you are a Chinese group tourist holding a group-tourist visa to Japan and meet certain requirements (e.g. using a travel agency that only attracts Chinese group tourists), you are permitted to enter Korea without a visa.
If you hold an e-visa to Australia and a visa label is attached to your passport, you are permitted to enter Korea without a visa. However, if the label is not on your passport, you are granted no-visa entry only when you depart from Australia and transit through Korea.
3. Condition of Permission
You have a confirmed onward flight ticket for departure within 30 days after entering Korea, in addition to having no record of criminal offence such as overstay in the countries mentioned above
You had not been denied entry into Korea, or had not been fined more than 5 million KRW or not have been forcibly evicted or received a deportation order for violation of law in the past 3 years preceding the date of entry
You stayed no more than 3 days in the country of transit or stopover. (But, Not applicable when the person is returning to the immediate previous point of departure(country of transit or stopover)
For holders of U.S., Canada, Australia, or New Zealand e-visas, a visa sticker must be attached to the passport; if unattached, visa free entry will be applicable when departing from the 4 countries mentioned above bound for the country of origin or a third country.
(Source accurate as of 2020-02-03, I fixed few typos of the sources on my own. [Note that the quote is not licensed under CC BY 4.0. Copyright by Republic of Korea Government.])