The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by the English biologist Thomas Henry Huxley that separates the biogeographical realms of Asia and Wallacea, a transitional zone between Asia and Australia known as the Indo-Australian Archipelago. To the west of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origins is present. Wallace noticed this clear division in both land mammals and birds during his travels through the East Indies in the 19th century.

The line runs through Indonesia, between Borneo and Sulawesi (Celebes), and through the Lombok Strait between Bali and Lombok, where the distance is strikingly small, about 35 kilometers (22 mi), but enough for a contrast in species present in each island. The complex biogeography of the Indo-Australian Archipelago is a result of its location at the merging point of four major tectonic plates and other semi-isolated microplates in combination with ancient sea levels. Those caused the isolation of different taxonomic groups in islands relatively close to each other. Wallace’s Line is one of the many boundaries drawn by naturalists and biologists since the mid-1800s intended to delineate the distribution constraints of the fauna and flora of the Archipelago.