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ABSTRACT Prorocentrumcf.balticumwas the provisional designation assigned to strains of a small, pelagic, mixoplanktonic dinoflagellate found to produce carbon‐rich mucilage‐based prey capture devices, termed “mucospheres.” Here we characterize the morphology and phylogeny of the strains, describe them asProrocentrum insidiosumsp. nov., and discuss common morphological features among the six species of the phylogenetically definedP. cordatumgroup. Cells ofP. insidiosumsp. nov. were round to slightly ovate in lateral view, 12–16 μm long and 8–15 μm deep, and laterally compressed. Scanning electron microscopy revealed the thecal plates were densely ornamented with short spines and there were two size classes of pores irregularly distributed across both plates, and a row of two to four large round pores in apical‐ventral position on the right thecal plate. The periflagellar area consisted of eight platelets, and there were two prominent wing‐like apical projections in the form of a double layered curved structure on platelet 1 with additional projections on most other platelets except platelet 4.Prorocentrum insidiosumsp. nov. is distinct from all genetically represented species within the genus and possesses a unique combination of morphological features differentiating it from other protologues of smallProrocentrumspecies.