90 daysEntry, Exit and Visa RequirementsSpain is a party to the Schengen Agreement. This means that U.S. citizens may enter Spain for up to 90 days for tourism or business without a visa. Your passport should be valid for at least three months beyond the period of stay.
Is 6 nights in Barcelona too long?
With six or seven nights, you will have time to see most of the city, as well as being able to see other interesting places near Barcelona. If you intend to do one or several day trips from Barcelona during your stay, you have an incredibly wide range of options.