Inquisitor lassulus is a species of sea snail, a marine gastropod mollusk in the family Pseudomelatomidae.

Its colour is buff, chequered with ochre-red, disposed irregularly, though alternate segments of red and buff on the subsutural cord seems a constant feature; usually the snout, the fasciole, and a narrow supra-basal zone are buff, while the intercostal spaces and a broad basal zone are red.

Sculpture:—A prominent undulating cord runs beneath the suture, and is followed by a rather narrow fasciole with a median groove.

The ribs may, or may not, swell at irregular intervals into varices—they are stout, perpendicular, discontinuous, persistent on the body whorl, and reach to the base; they are set at about eleven to the whorl.

The spirals are flat-topped cords, their width apart, more prominent in the interspaces than on the ribs Their interstices and sometimes themselves are crossed by a secondary sculpture of fine radial threads, excluding the subsutural spiral.