This article is from the Puerto Rico FAQ, by Zeydy Ortiz Laureano firstname.lastname@example.org with numerous contributions by others.
Traveling to Puerto Rico is as if you were going to any other state
within the USA. You will only need a passport (and a visa if you are
not a citizen of the USA) if you are coming from another country.
At the airports in Puerto Rico, your luggage will be inspected by the
U.S. Department of Agriculture to make sure you are not carrying
prohibited fruits and plants to the mainland. Travelers carrying
undeclared prohibited items will be fined on the spot. If you want a
copy of what is and is not permitted back on the mainland, write to
the U.S. Agriculture Department, Animal and Plant Health Inspection
Service, Room G-110, Federal Building, Hyattsville, MD 20782.
Consult your travel agent for more details.