A marine epiphytic diatom Cocconeis convexa Giffen was obtained on seaweeds collected from the tidal zone, and its morphology has been investigated using the light microscopy (LM) and the electron microscopy (EM) with the bleaching method. The valves are elliptic or rhombic. The valve face of the raphid valve (RV) is concave and that of the araphid valve (AV) is complementary and strongly convex. The single plastid is flat and C-shaped. The raphe on the RV is straight, and terminates in slightly deflected belietoglossae internally. The striae consist of small, round areolae and are radiate and uniseriate. The hymenes, with perforations arranged in a centric array, are located near the internal openings of the areolae. On the AV, the striae consist of several alveoli. Each alveolus is occluded by a hymene that has perforations arranged in a parallel array and located near the outer surface. The cingulum of AV consists of three open bands without fimbriae; a valvocopula and two bands with a ligula.