P. marmoratus has a square carapace 22–36 millimetres (0.87–1.42 in) long, which is dark violet brown with marbling in yellow.

[2] It is capable of very rapid movements, and it uses this ability to dart into crevices, making it difficult to catch.

[6] P. marmoratus is an omnivore, but not an opportunist; similar proportions of algae and animals are consumed however abundant they are in the habitat.

[7] On more sheltered shores, P. grapsus eats fewer mussels, but compensates with a greater consumption of barnacles.

This results in high levels of gene flow between populations, and allows the species to rapidly colonise new areas.