Shell up to 61 mm in length, thin but solid, shape fusiform with long siphonal canal. Spire angle 47°-50° (knobs included), 37°-43° when using the suture as measuring point. Suture adpressed to preceding whorl, forming a subsutural concavity. Colour orange with 1-4 whitish spiral bands on the penultimate whorl, 5-10 on the body whorl. Protoconch smooth and globose, 1.3-1.4 mm in diameter, consisting of 1.25-1.50 whorls, first whorl deviated from coiling axis by about 45°. Teleoconch consisting of 6 or 7 whorls. Adapical whorls with 2 primary spiral cords, 3 on penultimate and 8-13 on body whorl (carina, 0-3 above and 7-9 below shoulder). Carina and 2 or 3 spirals below most prominent, producing a well-developed knob when crossing axial ribs. One secondary intercalated spiral cord appears at second or third teleoconch whorl. On penultimate and body whorl 4-7 (rarely 3) secondary spiral cords of equal strength between each pair of primary cords, interspaces narrow. 20-35 weak spiral threads on siphonal canal, adapical ones alternating well-developed and fine. 7-9 broad axial ribs on first teleoconch whorl, 8-10 on body whorl, running from just above shoulder down to about halfway the base, producing 3 or 4 strong knobs. Aperture ovate, pinched at both ends, whitish in colour with narrow orange-brown border along inner lip. Inner side with 12-17 lirae, adapically arranged 2 by 2. The single intact specimen has a smooth lip. Columellar callus thin, smooth and glossy, while to pale orange-brown in colour, adapically with a low columellar fold. Siphonal canal straight, without torsion.
Operculum corneus, light to dark brown, above rounded, lower side pointed, with terminal nucleus.

S. sitanius differs from other known recent Serratifusus species by the inverse pattern (white spirals on coloured background instead of coloured spirals on whitish background), by the orange-brown colour and by differences in spiral sculpture (3-7 secondary spiral cords of about equal strength). S. sitanius is variable in shape. The shell may have strong axial knobs (as in holotype) or be rather rounded (as paratypes), a situation also observed in S. lineatus and S. excelens.

S. sitanius is known from off northern New Caledonia. Bathymetrie range, empty shells only, 460-600 m.