During a survey of the epizoic diatom flora on carapaces of loggerhead sea turtles (Caretta caretta) from the Adriatic Sea, a relatively small population of an unknown amphoroid species was observed, presenting some morphological similarities with the genera Lunella and Catenula. Due to the small size and the lack of distinct structures visible in LM, the observations of its ultrastructure under very high magnification in the scanning electron microscopy (SEM) proved to be critical in taxon delineation and indicated that it should be separated as a new Catenula species. The new species is characterized by very small, dorsiventral valves, striae composed of 1–3 small, marginally placed areolae, a very simple raphe structure lacking terminal raphe fissures, straight simple central raphe endings, the absence of helictoglossa and a distinct central nodule, the latter often discernible in LM. Typical ghost striae, reported to be one of the main morphological features of the genus Catenula, are very faintly present and only visible in the valve interior. The new species is illustrated using both LM and SEM observations and compared with all other known Catenula and Lunella species.