U.S.Citizens are entitled to drive in Portugal with their U.S. issued driver's license for a period no to exceed 185 days, provided they are not legal residents.
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Americans are not required by law to obtain an international driving permit to drive a rental car in Portugal.
International Driving Permit (if you have one) Passport. Car documents (like the registration, proof of ownership, and vehicle inspection report) Proof of insurance (in Portugal this is represented by a sticker on the lower right corner of the windshield as well as a certificate of insurance)
Cases in which you have to exchange the foreign license for a Portuguese one. If you have a driving licence issued in a country with a bilateral agreement with Portugal, you can drive in Portugal with your foreign driving licence for 185 days (about 6 months), provided you are not a resident.
Portuguese traffic lights follow the conventions of the Vienna agreement and so should be familiar to most drivers. You cannot turn right on a red light unless indicated and you should also be aware that many local drivers run red lights.
It's a good idea to get one if you'll be driving in Austria, Bosnia-Herzegovina, Croatia, Greece, Hungary, Italy, Poland, Romania, Slovenia, Slovakia, or Spain — countries where you're technically required to carry a permit.