Kaua i (in standard Hawaiian ; in Kaua i-Ni ihau dialect, [tauˈaʔi]; usually spelled Kauai outside the Hawaiian Islands and or /kaʊˈɑɪ/ ) is the oldest of the main Hawaiian Islands. With an area of , it is the fourth largest of the main islands in the Hawaiian archipelago and the 21st largest island in the United States. Known also as the "Garden Isle", Kaua i lies 105 miles (170 kilometers) across the Kaua i Channel, northwest of O ahu. Of volcanic origin, the highest peak on this mountainous island is Kawaikini at . The second highest peak is Mount Wai ale ale near the center of the island, above sea level. One of the wettest spots on Earth, with an annual average rainfall of 460 inches (11,700 mm), is located on the east side of Mount Wai ale ale. The high annual rainfall has eroded deep valleys in the central mountains, carving out canyons with many scenic waterfalls.

There is no known meaning behind the name of Kaua i. Native Hawaiian tradition indicates the name's origin in the legend of Hawai iloa — the Polynesian navigator attributed with discovery of the Hawaiian Islands. The story relates how he named the island of Kaua i after a favorite son; therefore a possible translation of Kaua i is "place around the neck", meaning how a father would carry a favorite child. In South Africa, a health food and drink franchise is named after the island .

The United States Census Bureau defines Kaua i as Census Tracts 401 through 409 of Kaua i County, Hawai i, which is all of the county except for the islands of Kaʻula, Lehua, and Ni ihau. The 2000 census population of Kaua i (the island) was 58,303.