Gerastos is a genus of proetid trilobite in the family Proetidae that lived between the Pragian and Eifelian of the Lower-Middle Devonian, spanning approximately 21 million years.

[2] Gerastos has been found in the Devonian of Belgium, Germany, the Czech Republic and Morocco.

[2] Gerastos has holochroal eyes situated to the side of its enlarged glabella region of the cephalon.

The surface of the cephalon can be smooth or covered with pustules.

[3][4] Length rarely exceeds 3 cm.