Niʻihau or Niihau is the smallest of the inhabited Hawaiian Islands in the U.S. state of Hawaiʻi, having an area of 69.5 square miles (180 km2).

Known as the "Forbidden Isle," Niʻihau lies 17.5 miles (15.2 nmi; 28.2 km) across the Kaulakahi Channel, southwest of Kauaʻi. The United States Census Bureau defines Niʻihau (with the neighboring small island of Lehua) as Census Tract 410 of Kauai County, Hawaii. Its 2000 census population was 160; as of June 2009, the population was 130.

In 1864, the island was purchased from the Kingdom of Hawaii by the Robinson family and has been privately owned since that time. It is currently run by Bruce and Keith Robinson. Niʻihau is generally off-limits to all but relatives of the island's owners, U.S. Navy personnel, government officials and invited guests.

Tourists are able to visit the island through a limited number of supervised tours, including beach, diving, hiking, and hunting safaris. Niʻihau is also referred to as the "Mystery Island," or the "Distant Isle."

The island is historically notable as the location for the Niʻihau Incident, in which a Japanese fighter pilot crashed on the island and terrorized its residents for about a week following the World War II attack on Pearl Harbor.

Commercially, the people of Niʻihau are known internationally for their gemlike lei pūpū (shell lei) craftsmanship.