A new marine monoraphid diatom species, Planothidium pseudolinkei sp. nov., is described from the coast of Guangxi, China. The detailed morphology of this epipsammic diatom is studied by using both light and scanning electron microscopy. P. pseudolinkei differs from congeners by a combination of morphological features including capitate apices, multiseriate striae, a small central area on the raphe valve and an oblong sinus on the rapheless valve. Ecological preferences of Planothidium are also briefly discussed.