Pachymatisma johnstonia is a species of sponge belonging to the family Geodiidae.

A species of the north-eastern 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.

This article about a demosponge is a stub.