Elachista platysma is a moth of the Elachistidae family. It is found in Australia. The wingspan is 7.3–9 mm for males and 7.9-8.7 mm for females. The forewings are bluish grey and the hindwings are grey. The larvae feed on Lepidosperma tuberculatum and other Lepidosperma species. They mine the leaves of their host plant. Initially, the mine is straight and runs along the edge of the leaf. Later, it widens and turns downwards. The mine is filled with frass, except the distal 20 mm. Pupation takes place outside of the mine on a leaf of the host plant.