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

It is only known from rocky subtidal habitats around Kawau Island off the North Island of New Zealand.

This is a thickly encrusting sponge with a very firm texture, growing in loosely attached patches up to 7 cm across.

The granular outer layer is brilliant yellow-orange with a rather darker interior carrying many symbiont foraminifera.

It is often infested with the parasitic amphipod Polycheria antarctica.