A new genus and species, Platyprotus phyllosoma, of munnopsidid isopod is described from Enderby Land, Antarctica. Platyprotus differs from all other munnopsidids, except Microprotus Richardson, 1910 and some species in Storthyngurella Malyutina, 1999, in having fully ambulatory pereopods 5–7 without natatory setae. Platyprotus differs from those genera primarily in body shape, lack of marginal spines on pereonites 5–7 and pleotelson, lack of a preanal ridge, and in possessing highly derived spatulate setae on article 3 of the mandibular palp. All three genera are part of he wider ‘Storthyngura’ clade. If the present concept of Storthyngurella is confirmed, ambulatory pereopods 5–7 have been derived independently more than once within hat clade. The family name is emended to Munnopsididae Hansen, 1916.