Yes, in 2026, all U.S. citizens are required to present a valid U.S. passport to enter the Federation of Saint Kitts and Nevis by air or sea. The passport must have at least six months of validity remaining and one blank page for an entry stamp. In addition to a passport, all travelers must complete an Online Immigration/Customs form (eTA) before arrival at a cost of approximately US $17.00. While cruise passengers may technically use other Western Hemisphere Travel Initiative (WHTI) compliant documents (like a birth certificate and ID) to enter on a "closed-loop" cruise, the U.S. State Department strongly recommends a passport. This is crucial in case of an unforeseen emergency—such as a medical evacuation or a missed ship—that would require you to return to the U.S. by air, which is impossible without a valid passport book.