It is a US territory but since it's a port of entry you have to do customs. You will go through Customs and Immigration upon your return to the mainland from the USVI, not upon your arrival to the USVI from the mainland. You will also go through immigration if you make a trip to the BVI. Hope that helps!

Yes, you must go through a U.S. Customs and Border Protection (CBP) inspection when traveling from the U.S. Virgin Islands (USVI) to the U.S. mainland, even though the USVI is a U.S. territory. This is because the islands are considered a "separate customs territory" to allow them to maintain their own unique duty-free status and tax laws. While U.S. citizens do not need a passport for this journey (a government-issued photo ID like a driver's license is sufficient), you will be required to fill out a customs declaration form. The "benefit" for travelers is a significantly higher duty-free exemption—up to $1,600 per person—which is double the standard $800 allowance for most international destinations. You must declare all purchases, particularly tobacco and alcohol, and the CBP officers will conduct a brief interview and potentially scan your luggage to ensure you are not bringing back prohibited agricultural items or exceeding your generous duty-free limits before you board your flight to the mainland.