Stylidium longicornu is a dicotyledonous plant that belongs to the genus Stylidium (family Stylidiaceae). It is an annual plant that grows from 10 to 30 cm tall. The obovate to spathulate leaves form a basal rosettes around the stem. The leaves are around 4–6 mm long. Inflorescences are unbranched racemes and produce flowers that are violet with white at the base and bloom from June to August in their native range. S. longicornu is endemic to the Kimberley region in Western Australia. Its habitat is recorded as being sand flats near sandstone. It grows in the presence of S. lobuliflorum, Rhynchospora, and Leptocarpus.