Polymastia boletiformis forms small cushions or broad-based masses of tissue up to 10 cm (4 in) in diameter, adhering firmly to the substrate.

The mesohyl, a jelly-like layer in the body wall, is stiffened by mineral spicules, large and small needle-like megascleres, but there are no microscleres.

The texture of this sponge is smooth and firm but supple, and the colour is yellowish-grey, yellow or orange.

Its typical habitat is on the top of boulders and on up-facing rocks covered with sediment, in association with a mat of bryozoans and hydroids.

[4] Secondary metabolites found in Polymastia boletiformis show antimicrobial activity; extracts from the sponge contain novel steroid/amino acid conjugates.