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U.S. citizens require a passport to enter Grenada, but they do not require a visa for stays up to three months. There is an airport departure fee of US $20 for adults and US $10 for children between the ages of five and twelve.

U.S. citizens should take special care to secure their passports while traveling as it can be time-consuming and difficult to acquire new proof of citizenship to facilitate return travel should the passport be lost or stolen.
Prior to departure from the U.S., sea travelers must have a valid U.S. passport or other original proof of U.S. citizenship, such as a certified U.S. birth certificate with a government-issued photo ID.
While a U.S. passport is not mandatory for sea travel, it is recommended since it is a more readily recognized form of positive proof of citizenship.
The U.S. Consulate General recommends traveling with a valid U.S. passport to avoid delays or misunderstandings. A lost or stolen passport is also easier to replace when outside of the United States than other evidence of citizenship.
For additional information concerning entry/exit requirements, travelers may contact the Embassy of Grenada, 1701 New Hampshire Avenue, N.W., Washington, D.C. 20009, telephone: (202) 265-2561, Fax: (202) 265-2468: e-mail:
.(JavaScript must be enabled to view this email address) , or the Consulate of Grenada in New York.
Sea travelers must have a valid U.S. passport (or other original proof of U.S. citizenship, such as a certified U.S. birth certificate with a government-issued photo ID).
A valid Canadian passport should be carried for all visits outside Canada, regardless of the destination or means of transportation. For travel to Grenada, Canadians are not required to carry a passport.
They are only required to be in possession of valid official photo identification (such as a driver's license) and proof of citizenship (such as a birth certificate).
However, travelers without a valid passport may encounter difficulties upon entry and departure, especially if their itinerary changes and if they must transit the U.S. or another country. All travelers to Grenada must also be in possession of an onward or return ticket.
These requirements are subject to change at any time. It is the traveler's responsibility to check with the country's embassy or its consulates for up-to-date information.
Sources:
U.S. State Department
Canadian Consular Affairs