Idotea balthica has a broad geographical distribution, having been recorded from the Belgian Exclusive Economic Zone, Great Britain, Cobscook Bay, Dutch Exclusive Economic Zone, European waters, Greek Exclusive Economic Zone, Gulf of Maine, Knokke, North West Atlantic, Red Sea, Voordelta, West Coast of Norway, Wimereux and the Black Sea.
The male is larger than female, and can reach 4 centimetres (1.6 in) long. The carapace is yellow, brown or green with small white spots; the female is usually darker. The species can be distinguished from other idoteids by the shape of the telson, which is dorsally keeled with straight sides in I. balthica, and has a distinct spine at the end.
- Gary Poore & Marilyn Schotte. "Idotea balthica (Pallas, 1772)". In M. Schotte, C. B. Boyko, N. L. Bruce, G. C. B. Poore, S. Taiti & G. D. F. Wilson. World Marine, Freshwater and Terrestrial Isopod Crustaceans database. World Register of Marine Species. Retrieved January 1, 2012.
- Kåre Telsnes. "Baltic Isopod – Idotea balthica". The Marine Flora & Fauna of Norway. Retrieved January 1, 2012.