Two new species of Dytiscidae are reported from Hainan Island, China: Neptosternus yanbini Bian and Ji, new species and Microdytes wewalkai Bian and Ji, new species. Neptosternus yanbini is similar to N. pocsi Satô, 1972 in the color pattern of the elytra, but can be easily distinguished by the different shape of median lobe of aedeagus. Microdytes wewalkai is similar to M. jaechi Wewalka, but can be distinguished from it by its smaller size, darker body color, slimmer median lobe in lateral view, and is without two longitudinal rows of stronger punctures on the elytral disk.