Niihau (/niha/; Hawaiian: (niihw)) is the westernmost and seventh largest inhabited island in Hawaii. It is 17.5 miles (28.2 km) southwest of Kauai across the Kaulakahi Channel. Its area is 69.5 square miles (180 km). Several intermittent playa lakes provide wetland habitats for the Hawaiian coot, the black-winged stilt, and the Hawaiian duck. The island is designated as critical habitat for Brighamia insignis, an endemic and endangered species of Hawaiian lobelioid. The United States Census Bureau defines Niihau and the neighboring island and State Seabird Sanctuary of Lehua as Census Tract 410 of Kauai County, Hawaii. Its 2000 census population was 160; Its 2010 census population was 170.

Can we conclude that is niihau a part of the united states?
yes