A new benthic marine dinoflagellate, Bysmatrum granulosum Ten-Hage, Turquet, Quod & Couté, sp. nov., was obtained from sediment and coral samples from sites of La Réunion Island (SW Indian Ocean). This new species is described and illustrated by light and scanning electron micrographs. Cells are 40–50 μm long and 40–46 μm wide. The epitheca is conical and smaller than the hypotheca, which is trapezoidal with convex sides. Plate tabulation is typical for the genus (P0, X, 4′, 3a, 7′′, 6c, 5s, 5′′′, 2′′′′); epithecal plates 3′ and 4′′ separate the intercalary plates 2a and 3a. Thecal plates are perforated by pores and covered by both small and minute wart-like projections, linearly arranged, radiating from the apical pore. This new species differs from the three others of the genus Bysmatrum in the following characters: cell shape and size, size of the apical pore complex and features of thecal plates (plate shapes and ornamentations).