Niihau 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).

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.

Elizabeth Sinclair purchased Niʻihau in 1864 for $10,000 from the Kingdom of Hawaii and private ownership passed on to her descendants, the Robinson family. During World War II, the island was the site of the Niihau Incident: A Japanese navy fighter pilot crashed on the island and terrorized its residents for a week after the attack on Pearl Harbor.

The people of Niihau speak Hawaiian as a primary language. The island is generally off-limits to all but the Robinson family and their relatives, U.S. Navy personnel, government officials, and invited guests, giving it the nickname "The Forbidden Isle".

Beginning in 1987, a limited number of supervised activity tours and hunting safaris have opened to tourists. The island is currently managed by brothers Bruce Robinson and Keith Robinson.

The island is about 6 million years old, making it geologically older than the 5.8 -million-year-old neighboring island of Kauai to the northeast. Niihau consists of one extinct volcano that had a large landslide to the east.

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