Diamond Head is a volcanic tuff cone on the Hawaiian island of Oʻahu. It is known to Hawaiians as Lēʻahi (pronounced [leːˈʔɐhi]), which is most likely derived from lae (browridge, promontory) plus ʻahi (tuna) because the shape of the ridgeline resembles the shape of a tuna's dorsal fin. Its English name was given by British sailors in the 19th century, who named it for the calcite crystals on the adjacent beach.

Diamond Head is part of the system of cones, vents, and their associated eruption flows that are collectively known to geologists as the Honolulu Volcanic Series, formed by renewed eruptions from the Koʻolau Volcano that took place long after the volcano formed and had gone dormant. These eruptive events created many of Oʻahu's well-known landmarks, including Punchbowl Crater, Hanauma Bay, Koko Head, and Mānana Island.

Like the rest of the Honolulu Volcanic Series, Diamond Head is much younger than the main mass of the Koʻolau Mountain Range. While the Koʻolau Range is about 2.6 million years old, Diamond Head is estimated to be about 400,000 to 500,000 years old.

Known as Lēʻahi in Hawaiian, the mountain was given the name Diamond Hill in 1825 by British sailors who discovered sparkling volcanic calcite crystals in the sand and mistook them for diamonds. This is reflected in another local name, Kaimana Hila. The name later became Diamond Head, with head being shortened from headland.

The interior and adjacent exterior areas were the home to Fort Ruger, the first United States military reservation on Hawaii. Only Battery 407, a National Guard emergency operations center, and Birkhimer Tunnel, the Hawaii State Civil Defense Headquarters (HI-EMA), remain in use in the crater. An FAA air traffic control center was in operation from 1963 to 2002.

Diamond Head is an iconic landmark overlooking Honolulu, and is featured as a backdrop to Waikīkī in numerous movies and TV shows set in Hawaii. It is both a State Monument and, since 1968, a U.S. National Natural Landmark. While part of it is closed to the public and serves as a platform for antennas used by the U.S. government, a 0.8 mile trail open to tourists leads to the summit from the Visitors Center at the crater floor. Access to the trail for non-Hawaii residents requires reservation online beforehand and a small fee. The trail was originally built in 1908 as part of the U.S. Army Coastal Defense System. The trail rises 560 feet and includes a series of steep paths, stairs, and tunnels. At the summit are several bunkers which were used as a control station to direct artillery fire in case of war. The view from the top and its proximity to Honolulu's resort hotels and beaches makes the summit a popular tourist destination.

In 1968, Diamond Head was declared a National Natural Landmark. The crater, also called Diamond Head Lookout, was used as a strategic military lookout in the early 1900s. Spanning over 475 acres (190 ha) (including the crater's interior and outer slopes), it served as an effective defensive lookout because it provides panoramic views of Waikiki and the south shore of Oahu.

The Diamond Head Lighthouse, a navigational lighthouse built in 1917 is directly adjacent to the crater's slopes. In addition, a few pillboxes are on Diamond Head's summit.