Gaius mainae is a species of mygalomorph spider in the Idiopidae family. It is endemic to Australia. It was described in 2018 by Australian arachnologists Michael Rix, Robert Raven and Mark Harvey.[1]
The species occurs in Western Australia in the Coolgardie and Mallee bioregions. The type locality is Grass Patch, some 80 km north of Esperance.[1]