Elachista bidens is a moth of the Elachistidae family that is found in Western Australia. The wingspan is about 10.6 millimetres (0.42 in) for males and 12.7 millimetres (0.50 in) for females. The forewings are blue. The hindwings are grey. The larvae feed on Lepidosperma tuberculatum. They mine the leaves of their host plant. The mine is straight, whitish and reaches a length of about 200 millimetres (7.9 in). Pupation takes place outside of the mine on a leaf of the host plant.