Pachymatisma johnstonia is a species of demosponge belonging to the family Geodiidae. A species of north east Atlantic coasts, this is a usually grey encrusting sponge with large prominent oscula and a pale yellow interior. The size and form depends largely on the extent of its exposure to waves. In heavily wave-exposed locations, it is usually small and thin but in more sheltered places can grow to over 50 cm across and 15 cm thick.