Do you need a passport? For US citizens the answer depends on how you travel, not just where

Puerto Rico, the US Virgin Islands, Guam, American Samoa, and the Northern Mariana Islands are US territories. A US citizen flying to any of them from the mainland is travelling domestically, and there is no immigration inspection of the kind a foreign trip involves.

The confusion is understandable. The flight is long, the destination feels international, and airlines sometimes ask for identification in a way that resembles a border check. None of that changes the legal position, and a passport is a convenience rather than a requirement.

Where it does become a requirement is if the itinerary touches somewhere that is not American on the way. A cruise calling at a foreign port, or a connection routed through another country, brings the international rules back into play for that leg regardless of where the trip started and ended.

From 7 May 2025, a state-issued licence or identification card that is not REAL ID compliant is no longer accepted at the airport checkpoint. Compliant cards carry a star in the upper corner, which is the quickest way to check the one in your wallet.

A US passport book and a US passport card are both on the list of alternatives TSA accepts, alongside trusted traveller cards including Global Entry, NEXUS, SENTRI and FAST, and Department of Defense identification. So a passport solves the domestic identification problem even where no passport is needed for the destination.

This is the sequence that goes wrong: someone establishes correctly that Puerto Rico needs no passport, leaves the passport at home, and arrives with a licence that has no star. The destination rule was applied correctly and the checkpoint rule was never considered.

Trusted traveller membership does not remove the identification requirement. Expedited screening changes the lane, not the document.

The passport card is a low-cost, limited-use document. It works for entering the United States from Canada, Mexico, Bermuda, and some Caribbean countries by land or sea, and it works as identification at a domestic airport checkpoint.

It is not accepted for international air travel. That single restriction is where the card's reputation for being confusing comes from, because the destinations it covers are precisely the ones people often fly to. A card that is perfect for driving to Canada is useless for flying to the same city.

For someone deciding between the two, the practical question is whether any international trip in the life of the document might involve a plane. If the answer is yes, or unknown, the book is the document that covers both. Holding both is common for people who cross a land border regularly and also fly abroad occasionally.

An emergency changes this calculation sharply. A family emergency abroad becomes a flight, not a drive, and a card holder in that position is applying for a book under time pressure.

A closed-loop cruise, meaning one that begins and ends at the same US port, is the case where a passport card or other accepted document can be enough, because the entry back into the United States is by sea.

The risk is what happens if the cruise stops being closed-loop. A medical situation, a missed sailing, or an itinerary change can leave a passenger needing to fly home from a foreign port, and that flight requires a passport book that the passenger did not bring.

This is why the conservative advice for cruises is the book even where the card is technically sufficient. The document requirement for the planned trip and the document requirement for the trip going wrong are not the same.