Cymatosyrinx idothea is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

(Original description) The small, thin, acute shell is rose pink and not polished.

The axial sculpture consists of (on the body whorl about a dozen) protractively oblique sigmoid riblets, faint on the base and practically absent from the fasciole, with subequal interspaces.

The anal sulcus is wide and deep, adjacent to the suture and with no subsutural callus.

The columella is stout, short, white and obliquely attenuated in front.