Polymastia fusca is a species of sea sponge belonging to the family Polymastiidae.

It is found in shallow subtidal habitats in the far north of North Island, New Zealand.

This firm-textured sponge grows up to 20 cm across, often encrusting its substrate.

The outer layer is very dark brown, contrasting with a much paler yellow or orange interior.

The whole surface is covered in papillae, those at the centre markedly taller than those at the periphery.