The Australian endemic castniid tribe Synemonini represents a substantial radiation of diurnal Lepidoptera, especially in the southern temperate areas of the continent. The tropical areas of northern Australia support considerably fewer species, however, much of the fauna is presently undescribed. Here we diagnose, describe and illustrate a new castniid, Synemon kimberleyensissp. nov., from the remote Kimberley region of northwestern Australia. Synemon kimberleyensissp. nov. is allopatric and closely related to S. phaeoptilaTurner, 1906 from the Top End, Wet Tropics and Cape York Peninsula, Australia. This pair of species is part of an ecological set of six Synemon species that are a characteristic component of the tropical savannah woodlands, which occur extensively across the monsoon tropics of northern Australia and, as far as is known, all specialize on Poaceae as their larval host plants, especially the genus Chrysopogon Trin.