The aganippine spiny trapdoor spiders of the genus Gaius Rainbow, 1914 are revised. Seven new species are described from Western Australia: G. aurora sp. nov., G. austini sp. nov., G. cooperi sp. nov., G. hueyi sp. nov., G. humphreysi sp. nov., G. mainae sp. nov. and G. tealei sp. nov. The type species, G. villosus Rainbow, 1914, is re-illustrated and re-diagnosed, and molecular data for six (of eight) species and six genes are analyzed with Bayesian methods. Species of Gaius are iconic denizens of the Western Australian inland arid zone, renowned for their large size and extreme longevity. We here document the known diversity and conservation status of these spiders, and summarize their unusual biology and phenology.