The Indo-Pacific, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, is a biogeographic region of Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia. It does not include the temperate and polar regions of the Indian and Pacific oceans, nor the Tropical Eastern Pacific, along the Pacific coast of the Americas, which is also a distinct marine realm.
The term is especially useful in marine biology, ichthyology, and similar fields, since many marine habitats are continuously connected from Madagascar to Japan and Oceania, and a number of species occur over that range, but are not found in the Atlantic Ocean.
The region has an exceptionally high species richness, with the world's highest species richness being found in at its heart in the Coral Triangle, and a remarkable gradient of decreasing species richness radiating outward in all directions. The region includes over 3,000 species of fish, compared with around 1,200 in the next richest marine region, the Western Atlantic, and around 500 species of reef building corals, compared with about 50 species in the Western Atlantic.
The term first appeared in academic use in oceanography and geopolitics. Scholarship has shown that the "Indo-Pacific" concept circulated in Weimar Germany, and spread to interwar Japan. German political oceanographers envisioned an "Indo-Pacific" comprising anticolonial India and republican China, as German allies, against "Euro-America". Since the late 2010s, the term "Indo-Pacific" has been increasingly used in geopolitical discourse. It also has a "symbiotic link" with the Quadrilateral Security Dialogue, or "Quad", an informal grouping between Australia, Japan, India, and the United States. It has been argued that the concept may lead to a change in popular "mental maps" of how the world is understood in strategic terms.