Contrary to a very popular misconception, Puerto Rico is, in fact, part of the United States. It's a U.S. territory, which means that although its citizens and government cannot vote on federal matters, the people of Puerto Rico are U.S. citizens and entitled to all the rights that citizenship entails. Puerto Rico's status as a territory also means that U.S. citizens and lawful permanent residents don't need a passport to visit, nor do Puerto Ricans need a passport to come and go from the rest of the United States. But, as always, there are a few things to be aware of before you book your travel plans.
Visiting Other Ports
If your itinerary includes only U.S. states and territories, you don't need to bring your passport – but if your itinerary even touches on a foreign port like Aruba, Dominica or the Dominican Republic, you and everyone else in your party, including children, will need a passport. You'll need to show that passport when you enter that port, then to re-enter the United States afterward. If you're a lawful permanent resident instead of a citizen traveling to any non-U.S. port, you may need to travel on the passport of your birth country – check the requirements of the destination country before you go. Either way, if your passport won't be valid for the duration of your stay and several months afterward, you should renew it well before your departure date.
Be Ready to Prove Identity and Citizenship
U.S. Customs and Border Protection recommends that even though you don't need your passport to go to and from Puerto Rico, you should still bring a government-issued photo ID and a copy of your birth certificate or, if relevant, your lawful permanent resident status. These are to prove your identity and citizenship which, not at all coincidentally, can both be proven by showing your passport or green card. So, even though a passport isn't strictly required, it's the easiest way to establish your identity.
Passport Requirements for non-U.S. Nationals
If you aren't a citizen or lawful permanent resident of the United States, you will need to show your passport when entering and exiting Puerto Rico, just as you would for any other part of the United States. All the other U.S. passport or visa requirements apply, too; check with your embassy in the United States, or with a U.S. embassy in your area for the most up-to-date information.