If you are planning to travel to Portugal on a tourist or business trip for a period not to exceed 90 days, you do not need to apply for a visa. You are required to be in possession of a U.S. passport, valid for a minimum of three months beyond the length of your stay.
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Yes, British citizens will need an ETIAS to visit Spain starting from 2023, when it becomes mandatory. The ETIAS will allow UK citizens multiple short trips of up to 90 days for tourism or business purposes, in periods of three years.