Genus Poulinea is a monospecific epizoic genus of marine diatoms found on sea turtles. It is a part of polyphyletic group of marine gomphonemoids, whose phylogenetical relationships are poorly understood. The aim of this study was to use a combined morphological and molecular approach to investigate Poulinea and related genera on loggerhead sea turtles from the Adriatic Sea. Firstly, the contribution of Poulinea cells in diatom epizoic communities was assessed using light microscope from seven loggerhead samples obtained in the period 1995-2004. Further on, morphology and morphometrics of Poulinea-like cells was assessed on scanning electron microscopy images taken from the first sample set with the addition of 31 loggerhead samples obtained from 2016 to 2019. Lastly, three strains of Poulinea were brought into laboratory cultures and information on their morphology and rbcL, psbC and SSU genes were obtained. Poulinea was recorded as a dominant taxon in the epizoic flora on the Adriatic loggerheads. However, four morphological groups of cells could be distinguished in the population of the Poulinea-like diatoms with only a single morphotype corresponding to the type species Poulinea lepidochelicola, while other groups showed higher variability and were generally more similar to Chelonicola spp. The phylogenetic analyses place all three cultured strains closely to other Poulinea lepidochelicola strains. This research offers new insights into the morphology and phylogeny of Poulinea and related taxa and sheds more light on the ecology of these small and poorly known epibionts.