Citizens of the United States (living in the United States) may typically visit Greece without a tourist visa for up to 90 days at a time. Business visas are not typically required either for stays less than 90 days, however, diplomatic visas are required. If you are a U.S. citizen but do not live in the USA, a Visa may be required. Also, If you have connecting flight(s) in other countries, check the Visa requirements for those countries. For additional information, visit the U.S. Embassy website for Athens, Greece...
Visitors from Greece
If you have relatives and friends visiting the U.S. from Greece, find their U.S. Visa requirements at the U.S. Department of State - Bureau of Consular Affairs website.
Passports that are valid for 3 months following your departure date from Greece are typically required. Make sure your passport has blank pages to accommodate arrival and departure stamps. U.S. Citizens may apply for a passport through the U.S. Department of State.