(From CBP)
What documents, identification, paperwork does a U.S. citizen or legal permanent resident (LPR) need to travel internationally?
If you are traveling in the Western Hemisphere (Canada, Mexico, Caribbean, Central and South America):
ALL persons*(see LPR and military exceptions below), including U.S. citizens, traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda are required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document.
As of June 1, 2009 all U.S. and Canadian citizens who are 16 and older traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), will be required to present a valid passport, passport card, Frequent Traveler card (NEXUS, SENTRI, FAST), Enhanced Driver’s License (for residents in states that issue them) or Military ID with travel orders (see below). U.S. and Canadian citizens who are 15 years old or younger may travel with just a copy of their birth certificate, as may teens between the ages of 16-18 if they are part of an adult supervised school, religious, cultural or athletic group.
For documentation requirements regarding cruises see our Frequently Asked Question “What Documents do I need for Cruises”
*Lawful Permanent Residents (Green card holders) do NOT need a passport to enter the United States, however you may need a passport to enter another country. Please contact the embassy of the foreign country you will be traveling to for their requirements.
*Military personnel traveling under orders may present photo id and orders. Family members must present a passport (with the exception of children 15 and younger arriving by land or sea).
Lawful permanent residents (LPR’s), refugees, and asylees will continue to be able to use their Alien Registration Card (Form I-551), issued by DHS, or the Travel Document issued to persons with refugee or asylee status to apply for entry to the United States. LPR’s may still need to present a passport for entry into a foreign destination. For this reason airlines may deny boarding to LPR’s traveling without a passport. NOTE: You may want to contact the airlines prior to traveling to confirm their documentation requirements.
If traveling from outside the Western Hemisphere, all U.S. citizens MUST present a passport, including children and infants.
For information about what documents are required for a U.S. resident to enter a foreign country, we advise you to contact the embassy or consulate general of the country you intend to travel to in order to find out what documents they require you to have for entry.