Amastra soror is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusc in the family Amastridae.
The length of the shell attains 11.6 mm.
(Original description) The shell is sinistral and conically turreted, comprising seven rounded whorls with a deeply impressed suture. The aperture is small and subovate, with a simple outer lip. The columella is very short, ending in a twisted plicate tooth.
The thin epidermis is dark corneous on the upper portion and light corneous below.
This species is endemic to Hawaii and occurs on Maui Island.