Gaius cooperi 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.
The species occurs in south-west Western Australia in the Avon Wheatbelt, Esperance Plains, Jarrah Forest and Mallee bioregions. The type locality is Forrestania, some 400 km east of Perth.