Notocytharella phaethusa is a species of sea snail, a marine gastropod mollusk in the family Mangeliidae.

(Original description) The minute shell has a very small turbinate brown protoconch of about 21⁄2 whorls, the latter part of which is feebly reticulately sculptured.

These threads on dead specimens show paler than the general surface.

On the base and the siphonal canal are about a dozen smaller close-set threads.

There is also some variation in stoutness and in the strength of the posterior thread which in a few specimens is stronger than the others giving a slight shoulder to the whorl.