Body very slender; length 4 to 7 times the width. Antenna 1 extending just beyond article 2 of the peduncle of antenna 2; like in Idotea chelipes , but unlike the other Idotea species, with a single aesthetasc, not a pair, at the distal end of the aesthetasc series. Antenna 2 very long and slender; peduncle rather longer than the flagellum, which is about a third of the body length. Cephalon subquadrate, eyes dorsolateral, prominent. Maxillipedal palp of four articles. Coxal plates very small, not extending to the posterior border of any pereional segment. Pereiopods long and slender, somewhat palmate distally; pereiopods 2 to 7 of adult males with pads of fine setae on their inner borders. Pleon of two complete segments, and one partial suture in the pleotelson. Pleotelson with the sides slightly concave anteriorly; posterior border concave and with a small median spine in young specimens. Uropods uniramous.
Males often recognizable from about 15 mm, ranging to over 40 mm in length; females rather smaller. Colour green or brown, with darker or lighter longitudinal stripes; adult female often darker than male, frequently with paler markings around the edges.
A sublittoral species, occasionally cast up on the shore and often found swimming near the water's edge on sand shores at low tide.
It is common in the Mediterranean, ranging south to Morocco and the Canaries and north as far as Denmark and Britain.