Buck Island Reef National Monument, or just Buck Island is a small, uninhabited, 176 acre (712,000 m²) island about 1.5 miles (2.4 km) north of the northeast coast of Saint Croix, U.S. Virgin Islands. The U.S. National Monument was created in 1961 and greatly expanded in 2001, over the bitter opposition of local fisherman. Most of the Monument area, which is administered by the National Park Service, is underwater. The marine ecosystem surrounding the island includes an elkhorn coral barrier reef that surrounds two-thirds of the island and provides cover for a great variety of reef fish. Endangered species that nest on the island include sea turtles and Least Terns. The clear waters, spectacular coral formations and abundance of marine life make Buck Island a favorite snorkeling destination. Snorkelers can enjoy a marked underwater trail near the eastern tip of Buck Island. The western edge of the island features a beautiful white-sand beach that makes it a popular gathering spot for local boaters.

There are actually two "Buck Islands." The one just north of St. Croix is the centerpiece of the National Monument. Buck Island National Wildlife Refuge occupies the one near St. Thomas.