Rhinocypha trimaculata Selys, 1853, is a very rare species, for which only four records have been previously published (twice in the 19^th century and twice in the 20^th century; most recently recorded in Assam in 1973). The species was rediscovered and photographically documented in Sylhet Division of Bangladesh in 2014. Later, in 2018, two male specimens were collected at the same site. All known earlier records of this species are documented and discussed. The male of R. trimaculata is redescribed and illustrated, including SEM photos of the penis. Diagnostic features, differentiating R. trimaculata and the related Rhinocypha ignipennis Selys, 1879, a montane species from north-eastern India and western Burma, are provided together with photographs of type specimens of both species. Habitats and behaviour of R. trimaculata are discussed. We note that this vulnerable or endangered species appears to be confined to small forest streams in lowland areas within a small range in northeastern India and Bangladesh and requires immediate conservation action to protect it.