With a total land area of just 44.97 sq mi (116.47 km²), Kahoolawe is the smallest of the eight main volcanic islands in the Hawaiian Islands.

The island is venerated as a kinolau or body form of the sea god Kanaloa. Symbolized by the squid or the octopus, Kanaloa was among the first group of spirits "spit out" by the gods. According to legend, he led a rebellion against his creators and was banished to the Underworld, where he rules as a god himself and a teacher of magic.

The island is uninhabited.

In the past, Kahoolawe was a beautiful green island with lots of vegetation, but the introduction of goats to the island in 1793, a gift from Captain George Vancouver of the British Royal Navy, led to uncontrolled grazing and substantial loss of soil through accelerated erosion. Other contributing factors included prolonged droughts and trade winds that blew away most of the topsoil, leaving behind red hardpan dirt.

During World War II and the following decades, Kahoolawe was used as a training ground and bombing range by U.S. Armed Forces, earning it the nickname "Target Island." One blast, which involved 500 tons of conventional TNT, created a crater on the island known as "Sailor Man's Cap" and may have cracked the island's caprock, causing groundwater to be lost into the ocean. After decades of protests, the island was transferred to the jurisdiction of the state of Hawaii in 1994, and the Hawaii State Legislature established the Kahoolawe Island Reserve to restore the island and its surrounding waters.

On January 6, 1976, more than 50 activists from across the Hawaiian islands made a coordinated effort to land on Kahoolawe, which was still being used by the Navy for bombing practice. Most of this group was intercepted by military craft, but "The Kahoolawe Nine" successfully evaded the barricade and landed on the island, where they protested peacefully before being detained and escorted back to Maui by federal marshals. Their protest brought attention to the destruction of the island.

When Hawaii achieved statehood in 1959, Daniel Inouye was elected as its first member of the U.S. House of Representatives. He was first elected to the U.S. Senate in 1962, where he served until his death. One of his most lasting legacies came in 1993, when he sponsored Title X of the Fiscal Year 1994 for the Department of Defense appropriation bill, directing the U.S. government to convey Kahoolawe and its surrounding waters back to the state of Hawaii.

Kahoolawe is relatively dry because of its location in the rain shadow of Maui's 10,023-foot-high (3,055 m) volcano, Haleakalā.

Under the rule of King Kamehameha III (1825-1854), the Kingdom of Hawaii replaced the death penalty with exile, and Kahoolawe became a men's penal colony. Food and water were scarce on the barren island. Faced with starvation, some prisoners attempted to swim across the channel to Maui. The law making the island a penal colony was repealed in 1853.

In 2020, a wildfire burned more than 30% of the island. Firefighters abandoned suppression efforts on the first day of the fire due to concerns about unexploded ordnance left on Kahoolawe by the U.S. Navy.