Shell size small for the genus; fusiform, elongate, slightly rounded whorls. Suture slightly impressed. Protoconch multispiral, about 2.3 smooth whorls. Teleoconch with about 3.2 whorls. No axial sculpture. Spiral sculpture consisting of a weak sub-sutural band on the last 1.5 teleoconch whorl and the basal cords on the body whorl, which ventrally range to the upper end of the columella. Outer lip clearly thickened on the outside, bearing continuation of the spiral cords. Inside of the outer lip not denticulate. Columellar callus thickened. Parietal callus very weak. Columella without denticles, except for one small denticle at the edge of the siphonal canal. Siphonal canal short, open, slightly recurved. Teleoconch whorls semi-translucent white, without pattern. Substural band greyish on penultimate whorl. Protoconch white. Aperture semi-translucent and columella white. Holotype height 3.2 mm.

Mitrella nainai looks close to Mitrella subtilicostata K. Monsecour & D. Monsecour, 2016. It differs in its smaller size, multispiral protoconch and the lack of the weak sculpture. It is found together with M. moorea, but M. nainai is smaller, has fewer teleoconch whorls and lacks the denticles on both lip and columella.

French Polynesia, Society Islands, Huahine and Moorea.