Antiplanes sanctiioannis is a species of sea snail, a marine gastropod mollusk in the family Pseudomelatomidae, the turrids.
The length of the shell varies between 20 mm and 40 mm; maximum diameter 14 mm.
The fusiform shell contains 10 slightly convex whorls. The shell is clothed with a smooth, thickish, olive epidermis. The shell is covered with very narrow spiral striae and incremental flexuous stripes. The dark reddish aperture is ovate and contracted below. It measures about 2/5 of the total length. The siphonal canal is broad and slightly oblique. The narrow outer lip has a wide sinuation above the middle. The columella is twisted. The operculum is unguiform.
This marine species occurs off he Pacific coast of northern Honshu, Japan; also in the Bering Sea.
