The flat leaf sponge (Isodictya grandis) is a species of marine demosponge in the family Isodictyidae.

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

It is an erect, thin-bladed sponge, with the blades narrowing to points at the tips.

Its texture is smooth but slightly hairy with small oscula on the blade edges.

[2][3] This sponge lives on rocky reefs from 15-37m.