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!

For U.S. citizens traveling from the mainland United States, you do not go through customs upon arrival in St. Thomas (U.S. Virgin Islands). Because it is a U.S. territory, the flight is treated similarly to a domestic trip, and you can enjoy your first moments on the island without any border hassle. However, the process is different when you depart the islands. Because the USVI is a "duty-free" zone outside the standard U.S. customs territory, you must clear U.S. Customs and Border Protection (CBP) at the Cyril E. King Airport before flying back to the mainland. During this departure process, you will be asked to declare any purchases. U.S. residents are entitled to a generous $1,600 duty-free allowance, which is significantly higher than the allowance for international trips. You will also undergo an Agriculture Scan for all luggage to ensure no prohibited plants or fruits are transported to the mainland. While a passport is not mandatory for U.S. citizens (a government-issued ID and birth certificate suffice), it remains the most efficient form of ID for speeding through the pre-clearance process.