The diversity and distribution of Pacific island iguanas were altered drastically following human colonization around 2800 years ago. A giant iguana recovered from archaeological sites in the Ha'apai group of islands, Kingdom of Tonga, became extinct within a century of human arrival. We describe this iguana as a new species of Brachylophus, a genus with two small arboreal species found today in Fiji (Brachylophus fasciatus, Brachylophus vitiensis) and parts of Tonga (Brachylophus fasciatus). Additional evidence suggests that B. fasciatus was probably introduced to Tonga (the type locality) by prehistoric people 2000 years after extinction of the giant form. Lapitiguana impensa, described in 2003 from Fiji by G. K. Pregill and T. H. Worthy was an even larger extinct iguana that also succumbed to human impact. The two living species are relicts of a much richer evolutionary history than previously known.