Eurydice are small, fast-swimming predators and scavengers with oval bodies and elongated pleotelsons. The peduncle of the antenna has four segments, whilst the second segment of the antennule is at a right angle to the first. The body is off-white with various darker markings, and the eyes are prominent.

E. affinis differs from the much more common E. pulchra in being somewhat smaller on average (males up to 5 mm, females up to 6 mm) and in the dark markings being restricted to the dorsal surface. The most significant characteristics is that the coxal plates of pereon somites 6 and 7 are not extended into points, and this feature should be examined closely to confirm identification.