Ancistrobasis monodon is a species of extremely small deep water sea snail, a marine gastropod mollusk in the family Seguenziidae.

[1][2] (Original description by Schepman) The height of the shell attains 4¾ mm.

Its upper margin is nearly straight, thin, thickened interiorly by a broad, white rib, with a few short plicae.

The basal margin is slightly convex, with a few inconspicuous plicae and a strong tooth near the columella, forming the end of an internal rib, winding round the columella.

[3] This marine species occurs off the Sulu Archipelago, Philippines, and off New Caledonia.