Yes, Americans are legally required to have an International Driving Permit (IDP) to drive or rent a car in Spain, in addition to their valid U.S. state driver's license. While some rental car agents at the counter might not ask for it during the pickup process, it is a mandatory legal requirement under Spanish law. If you are pulled over by the "Guardia Civil" or involved in an accident and cannot produce an IDP, you could face significant fines, and your insurance coverage might be rendered void because you are technically driving without the proper documentation. The IDP is essentially a translation of your license into multiple languages (including Spanish) and must be obtained before you leave the United States, typically through AAA. It is a simple, inexpensive document that lasts for one year, and carrying it ensures you are fully compliant with local regulations while navigating the scenic roads of the Iberian Peninsula.