No passports or visas are required for U.S. citizens entering Puerto Rico [1]. Those visiting the island from other countries must have the same documentation required to enter the United States. Visitors from the United Kingdom are required to have a British passport but do not need a visa unless their passports are endorsed with British Subject, British Dependent Territories Citizen, British Protected Person, British Overseas Citizen, or British National (Overseas) Citizen. A return ticket or proof of onward travel is necessary. Australian visitors must have a passport and can stay up to 90 days without a visa.
Links:
[1] http://www.moon.com/destinations/puerto-rico