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 Gascoyne, Little Sandy Desert, Murchison and Mallee bioregions. The type locality is Lorna Glen Station (now part of the Matuwa and Kurrara-Kurrara Indigenous Protected Area) about 840 km north-east of Perth.[1]