Amastra subobscura is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Amastridae.
The length of the shell attains 11.9 mm, its diameter 6 mm.
(Original description) The shell contains 6 whorls. The shell is minutely perforate, oblong-conic, and relatively thin with a slight sheen. The spire's outlines are nearly straight. The whorls of the protoconch are reddish-brown, with a smooth initial half-whorl followed by a coarsely costate whorl that is carinated just above the suture. The subsequent whorl is finely and densely costulate, with a subtle angularity near the lower suture.
The later whorls are convex, adorned with growth wrinkles and subplicate ridges just below the suture. They are pale fleshy-gray, partially covered by a very thin, smooth olivaceous cuticle, which is darkest on the latter half of the body whorl and absent near the aperture.
The aperture is pinkish-white, with the outer lip remaining unthickened. The parietal callus is whitish and moderately thick.
This species is endemic to Hawaii, occurring on Molokai Island.