Epiphytic diatoms on seaweeds and seagrasses were collected from tropical regions in the western Pacific Ocean (i.e., Siladen Island, Celebes Sea, Indonesia; Guam; Palau). Fifteen species of Diploneis are described in detail with light and scanning electron microscopy including six species new to science, D. cerebrum, D. claustra, D. craticula, D. crispa, D. rimosa and D. weissflogiopsis. Valve structure in Diploneis is particularly complicated, and two new distinctive structures are described for the first time: (1) central external raphe endings with flaps or rolled flaps and (2) internal globular papillae near the marginal ends of the virgae. In addition, we confirm that openings on the inner surface of the longitudinal canals are of taxonomic value. The structural complexities encountered in Diploneis require the study of a great many more taxa before a meaningful revision of the genus can be undertaken. Its diversity in warm waters is a rich resource for new Diploneis taxa to further our understanding of the genus.