Plagiostropha opalus is a species of sea snail, a marine gastropod mollusk in the family Drilliidae.

The shell has continuous longitudinal distant ribs.

The wide interstices are smooth or with revolving striae.

The shell is white with the interstices of the ribs sometimes more or less stained with brown.

[2] This species occurs in the demersal zone of the tropical Indo-Pacific off Fiji, New Caledonia and the Philippines.