Amastra seminigra is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Amastridae.
The length of the shell attains 21.2 mm, its diameter 10.3 mm.
(Original description) The shell contains 6ý whorls. The apical whorls are fleshy or purplish in color, transitioning to a light yellow hue on the last two whorls. These are partially or predominantly covered with a pitch-black cuticle, which may appear in scattered shreds or remain nearly intact, except near the aperture.
The interior of the shell is either white or faintly tinged with pink, adding a subtle contrast to its exterior.
This species is endemic to Hawaii, occurring on Oahu Island.