Angustassiminea castanea is a species of small operculate snail, a terrestrial gastropod mollusk or micromollusk in the family Assimineidae.
The length of the shell attains 5 mm, its diameter 2.5 mm.
(Original description in Latin) The solid shell is imperforate, pyramidally conical and chestnut-colored. It is very shiny, finely striate and spirally lined. The spire is acute. The shell has seven rather convex whorls, which are disjoined by a thin suture. The body whorl is faintly angled, convex at the base, and it nearly occupies half the length of the shell. The aperture ispear-shaped and acute above, with a straight parietal wall. The peristome (margin) is straight, with a arched basal margin. The inner lip is slightly thickened and reflected above.
This marine species has been found off Japan.