Entry/Exit Requirements for U.S. Citizens
Passports are required for U.S. citizens entering French Guiana, and should be valid for at least three months beyond the period of stay. Visas are generally not required for visitors planning to remain for 90 days or less. Anyone intending to stay more than 90 days must obtain the appropriate visa issued by the French Embassy or one of the French Consulates in the U.S., prior to departure for French Guiana. Visit the Embassy of France website for the most current visa information.
If you are traveling for reasons OTHER than business and tourism - employment, study, internship, etc. - you may require a French visa for that purpose, which you should obtain before you leave the United States.
Vaccination: U.S. Citizens travelling, or transiting through French Guiana must have proof of yellow fever vaccination.