Body length of female 2-2.5 mm, of male 1.2 mm. Body of female elongated, that of male considerably shorter and slightly constricted in the middle.
Propodus of female cheliped serrated or tuberculated on upper margin at insertion of dactylus (L. gracilis-detail). Inner margin of propodus distally with three or four sharp or round cusped teeth. Dactylus of cheliped serrated or tuberculated on upper margin, proximally at least.
Both sexes with a prominent ventral tubercle on each pleonite.
Uropods biramous. Endopodite long and two-articled, exopodite two articled, shorter than first article of endopodite.
This species is found predominantly in muddy sand, sometimes with small gravel or shell inclusions, at depths from 6-900 m.
It has been recorded from the northern North Sea, the east and west coasts of both Scotland and northern England, the Isle of Man and western Ireland. L. gracilis has a wide, nearly circumpolar distribution, having been recorded from Alaska via Greenland to Novaya Zemlya.
L. gracilis has frequently been confused with the similar Leptognathia longiremis . L. longiremis differs from L. gracilis in that the upper margin of the dactylus and propodus of the cheliped are smooth. Also the propodus of pereiopod 1 in L. longiremis is provided with spines along its posterior margin.