Shell small, 17 mm, high spired, biconic. Protoconch of two smooth whorls, 0.73 mm wide, 0.51 mm high. 8 teleoconch whorls. Suture distinct, slightly impressed. Upper third of whorls smooth, then strong axial ribs develop and continue to lower suture. Ribs narrow, reach maximum extent at midwhorl, then taper to lower suture. Ribs absent on body whorl. Numerous very fine spiral striae and axial growth lines on entire shell surface. Strong varix on left side of body whorl. Outer lip slightly thickened, strongly incurved, stromboid notch small but distinct. Sinus very deep, U shaped, on upper shoulder, at 30" angle to shell, margins elevated. Callus low, indistinct. Aperture narrow, deep, elongate. Columella broad, margin elevated. Anterior truncate, canal broad, medium depth, anterior notched. Shell colour uniform chalky white. Length 17,3 mm; Width 7,2 mm

Plagiostropha sinecosta is the fifth Australian species of the genus, the other four being P. opalus (Reeve, 1843), P. ebur (Reeve, 1843), P. quintupex Melvill, 1927, and P. flexus (Shuto, 1983). Plagiostropha sinecosta is clearly congeneric with the other four species on the basis of its biconic shape, strong varix on the left side of the body whorl, lack of strong ribbing on the body whorl, very deep sinus, reflected lip, narrow aperture and truncated body whorl. It differs from all other Plagiostropha in having small axial ribs but lacking the strong pentagonal axial ribbing.

Type locality
201 m depth, 210 km N of Port Hedland, Western Australia.