Mastigias siderea is a species of jellyfish in the family Mastigiidae.

There are about 60 marginal lappets and the surface of the bell is sculptured with deep radial furrows between these.

The eight mouth arms are as long as the diameter of the bell, terminating in club-shaped filaments.

The bell is pale yellowish-brown with white spots; these are small near the margin but larger over the ring canal.

The mouth arms are brown, spotted with white, and the terminal filaments are yellowish.