Two new monoraphid diatoms, Psammothidium hainanii Kociolek & Liu sp. nov. and Platessa guagnzhouae Liu & Kociolek sp. nov., are described. Both species were collected from southern China, from Hainan and Guangzhou province, respectively. Psammothidium hainanii is distinguished from other species by its unique outline and rectangular central area on the rapheless valve. Platessa guangzhouae has uniseriate areolae in both valves, similar to some Psammothidium species, but it also has features of a flat valve, terminal raphe fissures lacking, a hyaline ring round the margin, and one areola at the end of each stria. Based on these features we have placed it in the genus Platessa. The bow-tie shaped central area on the raphe valve and uniseriate areolae on the rapheless valve help to separate it from other Platessa species.