Niʻihau (Hawaiian: [ˈniʔiˈhɐw]), anglicized as Niihau ( NEE-(ee-)how), is the westernmost main and seventh largest inhabited island in Hawaii. It is 17.5 miles (28.2 km) southwest of Kauaʻi across the Kaulakahi Channel. Its area is 69.5 square miles (180 km²). Several intermittent playa lakes provide wetland habitats for the Hawaiian coot, the Hawaiian 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 Niʻihau and the neighboring island and State Seabird Sanctuary of Lehua as Census Tract 410 of Kauai County, Hawaii. Its 2000 census population was 160, most of whom are native Hawaiians; Its 2010 census population was 170. At the 2020 census, the population had fallen to 84.

Elizabeth Sinclair purchased Niʻihau in 1864 for US$10,000 (equivalent to about $170,000 in 2021) from the Kingdom of Hawaii. The island's private ownership passed on to her descendants, the Robinsons. During World War II, the island was the site of the Niʻihau incident, in which, following the attack on Pearl Harbor, a Japanese navy fighter pilot crashed on the island and received help from the island's residents of Japanese descent.

The island, known as "the Forbidden Isle", is off-limits to all outsiders except the Robinson family and their relatives, U.S. Navy personnel, government officials, and invited guests. From 1987 onward, a limited number of supervised activity tours and hunting safaris have opened to tourists. The island is currently managed by brothers Bruce and Keith Robinson. The people of Niʻihau are noted for their gemlike lei pūpū (shell lei) craftsmanship. They speak Hawaiian as a primary language.