Phalium glaucum, common name the grey bonnet or glaucus bonnet, is a species of large sea snail, a marine gastropod mollusk in the family Cassidae, the helmet snails and bonnet snails. This species lives on sandy bottoms with seagrass meadows, in intertidal and shallow subtidal areas to a depth of about 10 m. Shell of Phalium glaucum can reach a length of 60-147 millimetres.