Abstract The stramenopiles are a large and diverse group of eukaryotes that possess various lifestyles required to thrive in a broad array of environments. The stramenopiles branch with the alveolates, rhizarians, and telonemids, forming the supergroup TSAR. Here, we present a new genus and species of aquatic nanoflagellated stramenopile: Mediocremonas mediterraneus , a free-swimming heterotrophic predator. M . mediterraneus cell bodies measure between 2.0–4.0 μm in length and 1.2–3.7 μm in width, possessing two flagella and an oval body morphology. The growth and grazing rate of M . mediterraneus in batch cultures ranges from 0.68 to 1.83 d −1 and 1.99 to 5.38 bacteria/h, respectively. M . mediterraneus was found to be 93.9% phylogenetically similar with Developayella elegans and 94.7% with Develorapax marinus , two members within the class Developea. The phylogenetic position of the Developea and the ability of M . mediterraneus to remain in culture make it a good candidate for further genomic studies that could help us to better understand phagotrophy in marine systems as well as the transition from heterotrophy to phototrophy within the stramenopiles.