Superficially, Mysis relicta resembles a crayfish. Differences include a thin carapace not completely covering the thorax, with the last two segments exposed dorsally, and to a lesser degree laterally.
Appendages are long, thin, setose, biramous, and many segmented.
The first thoracic segment is fused with the head and bears a pair of maxillipeds. The remaining seven segments are distinct with the secondary maxillipeds on the second thoracic segment and a pair of swimming legs on each of the remaining five segments. Eyes are extremely large, compound, and stalked. The telson has a wide, bifurcated tip. The first nonindigenous introduction was proposed by Clements et al.