Kahoʻolawe is the smallest of the eight main volcanic islands of the Hawaiian Islands. Unpopulated, it lies about seven miles (11 km) southwest of Maui. The island is 11 mi (18 km) long by 6.0 mi (9.7 km) wide, with a total land area of 44.97 sq mi (116.47 km²). Its highest point is the crater of Lua Makika, at the summit of Puʻu Moaulanui, about 1,477 feet (450 m) above sea level.

Kahoʻolawe is relatively dry, with an average annual rainfall of less than 26 in (66 cm) resulting from a combination of being too low to generate much orographic precipitation from the northeastern trade winds and lying in the rain shadow of eastern Maui's 10,023-foot-high (3,055 m) volcano, Haleakalā. More than one quarter of Kahoʻolawe has been eroded down to saprolitic hardpan soil, largely on exposed surfaces near the summit.

Historically, Kahoʻolawe was always sparsely populated, due to its lack of fresh water. During World War II and the following decades, it was used as a training ground and bombing range by the Armed Forces of the United States. After decades of protests, the U.S. Navy ended live-fire training exercises on Kahoʻolawe in 1990, and the whole island was transferred to the jurisdiction of the state of Hawaii in 1994. The Hawaii State Legislature established the Kahoʻolawe Island Reserve to restore and to oversee the island and its surrounding waters. Today Kahoʻolawe can be used only for native Hawaiian cultural, spiritual, and subsistence purposes, fishing, environmental restoration, historic preservation, and education. It has no permanent residents.

Kahoʻolawe is an extinct shield volcano, which formed during the Pleistocene epoch. It was once connected to the island of Maui Nui before splitting off about 300,000 years ago. Most of the island is covered by basaltic lava flows. A caldera is located in the eastern part of the island. The last confirmed volcanic activity on the island occurred about one million years ago, though eruptions could have occurred about 10,000 years ago.

Kahoʻolawe experiences a semi-arid climate (Köppen BSh).

Sometime around the year 1000, Kahoʻolawe was settled by Polynesians, and small, temporary fishing communities were established along the coast. Some inland areas were cultivated. Puʻu Moiwi, a remnant cinder cone, is the location of the second-largest basalt quarry in Hawaiʻi, and this was mined for use in stone tools such as koʻi (adzes). Originally a dry forest environment with intermittent streams, the land changed to an open savanna of grassland and trees when inhabitants cleared vegetation for firewood and agriculture. Hawaiians built stone platforms for religious ceremonies, set rocks upright as shrines for successful fishing trips, and carved petroglyphs, or drawings, into the flat surfaces of rocks; these indicators of an earlier time can still be found on Kahoʻolawe. The island itself is venerated as a kinolau or body form of the sea god Kanaloa.

While it is not known how many people inhabited Kahoʻolawe, the lack of freshwater probably limited the population to a few hundred people. As many as 120 people might have once lived at Hakioawa, the largest settlement, which was located at the northeastern end of the island—facing Maui.

The U.S. Census Bureau defines Kahoʻolawe as Block Group 9, Census Tract 303.02 of Maui County, Hawaii.