Because St. Croix is part of the U.S. Virgin Islands (USVI) and is a U.S. territory, travel for U.S. citizens is considered domestic travel. You do not technically need a passport to enter St. Croix from the mainland United States. You only need a valid government-issued photo ID (like a REAL ID-compliant driver's license). However, a passport is highly recommended for two reasons: first, if you plan to visit nearby international islands like the British Virgin Islands, you will need it; second, you must pass through a customs and border protection check when returning to the mainland from the USVI, and having a passport makes this process significantly faster and more seamless. For non-U.S. citizens, standard U.S. visa and passport requirements apply as if you were entering any other part of the United States.