Ceramium cormacii is newly described based on both vegetative and tetrasporangial features of specimens growing on loggerhead sea turtles ( Caretta caretta ) from the Mediterranean Sea. The new species is characterised by both prostrate and erect axes (9 -12 mm high), dichotomously to pseudo-dichotomously branched with apices straight or slightly incurved, attached to the substratum by unicellular rhizoids often terminating in multicellular pads, incomplete cortication formed by one to three rows of acropetal cells, fi ve to six periaxial cells, and tetrahedral tetrasporangia produced by periaxial cells, strongly protruding and partially involucrated.