Acianthus pusillus, commonly known as small mosquito orchid, is a flowering plant in the orchid family Orchidaceae and is endemic to Australia. It is a terrestrial herb with a single, heart-shaped leaf and up to 18 small, translucent green or pinkish flowers with reddish marking and a green to reddish-purple labellum. It is widely distributed, growing in moist places from central-eastern Queensland, south through New South Wales and Victoria to South Australia and Tasmania.