During a survey of the epizoic diatom flora on carapaces of loggerhead sea turtles (Caretta caretta) from the Adriatic Sea, a fairly large population of an unknown marine Planothidium species was observed. Detailed morphological analysis based on scanning electron microscopy observations and comparison with a large number of known Planothidium species, resulted in the description of the new species as Planothidium kaetherobertianum sp. nov. The new species is characterized by a rapheless valve showing a rather broad axial area with irregular, shallow depressions, striae composed of only 1–2 rows of very small areolae. The raphevalve has biseriate striae with a short third row of areolae inserted between the two rows of areolae near the valve margins. The species shows some resemblance with the group of species around Planothidium delicatulum. The new species is illustrated using both LM and SEM observations and compared with other resembling marine and brackish Planothidium species.