Fossarina petterdi is a species of very small sea snail, a marine gastropod mollusc or micromollusk in the family Trochidae, the top snails.

[1] The shell grows to a length of 4 mm.

The shell is narrowly umbilicated, faintly spirally striate, with hardly visible longitudinal striae.

It is dark purplish black, with a few irregular white markings.

The interior is greenish black, showing the external white markings.