The State Department said the latest update reflected a low level of COVID-19 infections in South Korea.
A highly infectious coronavirus variant, Delta, is now dominating COVID-19 cases in India.
Costa Rica remains at a Level 4, indicating a "very high level of Covid-19".
However, the White House is setting up working groups with a series of foreign countries, including the United Kingdom, to determine how to safely restart global travel, with the issue due to be on the table at the G7 summit in Cornwall this weekend.
Such travel advice has been closely watched for clues on when the United States will end a ban imposed more than a year ago by former president Donald Trump on most visitors from the European Union and Britain. However, for Level 4 countries, if travel is an urgent need, the CDC recommends being fully vaccinated (two weeks after your final dose of the vaccine) before traveling.
The CDC still strongly recommends, or some destinations require, passengers to travel no sooner than 14 days after completing their vaccination.
A State Department official said the changes reflect recommendations issued a day earlier by the Centers for Disease Control and Prevention, which called on Americans to be fully vaccinated before traveling but said non-vaccinated people should avoid trips. "If you must travel to Costa Rica, make sure you are fully vaccinated before travel".
In the US, new infections are at the lowest levels since the pandemic began in March 2020. The recent improvement of cases in these geographies - especially Europe - has led airlines to lobby for a reopening.
It amounted to the first major relaxation of such warnings since the agency slapped a Level 4 travel advisory on the entire globe in March 2020 as the pandemic gained speed.
The State Department or the CDC did not provide reasons for the downward revision. According to data from the CDC, almost 50% of all people ages 12 and older in the United States have been fully vaccinated.
"This summer we have more direct flights from the USA than we've ever had, which shows that Americans want to travel to Greece", the Greek Prime Minister noted earlier in June.