Mediaster is a genus of starfish in the family Goniasteridae.

It was circumscribed in 1857 by William Stimpson for M. aequalis, the genus's type species.

[2] Its junior synonym is the genus Isaster, which was circumscribed in 1894 by Addison Emery Verrill for the species now known as M.

[3] Verrill himself synonymized the two genus names in 1899.

[4] As of 2017[update], the World Register of Marine Species lists the following species as being in the genus:[1]