Plectaster decanus, the mosaic sea star, is a species of sea star found off the south coast of Australia.

A group of raised yellow ridges covers its red upper surface and it is soft in texture.

Plectaster decanus is one of the few poisonous sea stars, and can cause numbness in humans if it is carried for any length of time.

[2] Plectaster decanus lives in rocky reefs off the southern coast of Australia.

[2] Plectaster decanus primarily feeds on sponges.