The brain sponge (Isodictya elastica) is a species of marine demosponge in the family Isodictyidae.

[1] This sponge is known from the west coast of South Africa to Port Elizabeth.

It has a smooth surface, covered with many scattered oscula.

It may be beige to pink in colour and grows in two forms: one massive and convoluted, rather like a brain, and the other with narrowing fingers.

[2][3] This sponge lives on rocky reefs subtidally to 25 metres (82 ft).