Simulium (Simulium) thimphuense sp. nov. is described from a pupa and a mature larva in Bhutan. This new species is placed in the S. multistriatum species-group, and is characterized by the pupal gill with eight thread-like filaments divergent basally at an acute angle when viewed laterally and the cocoon slipper-shaped with several small openings anterolaterally. Four species of Simulium (Simulium) are newly recorded from Bhutan: S. barraudi Puri in the S. multistriatum species-group, S. nodosum Puri in the S. nobile species-group, S. chiangmaiense Takaoka & Suzuki in the S. striatum species-group and S. himalayense Puri in the S. variegatum species-group. Our study increases the number of black fly species known from Bhutan from 18 to 23.