Do you need an International Driving Permit for Portugal?

Portugal does not legally require an IDP from US visitors staying under six months. Your US license is accepted on its own.

An IDP is still worth carrying as a translation aid, particularly outside Lisbon and Porto where an officer is less likely to read English.

An International Driving Permit is a translation of your licence into a standard multilingual booklet, issued under the 1949 and 1968 road traffic conventions. It is not a licence, and it is never valid on its own: you carry it with your own licence, and police expect to see both.

It is issued against the vehicle classes already on your home licence, and it never adds one. If your licence does not cover motorcycles, an IDP does not let you ride a scooter in Portugal; if it does not cover a heavier vehicle, the permit will not cover a camper or a minibus either. The permit restates what you already hold, in a form the local authorities can read.