A new species of ephemerellid mayfly, Cincticostellaebura sp. nov., is described based on larvae collected in a stream from Nan Province, Thailand. This new species is classified in the nigra complex of the genus Cincticostella based on morphological and COI phylogeny evidence. The new species is closely related to C.nigra (Uéno, 1928) and C.funki Martynov, Selvakumar, Palatov & Vasanth, 2021 based on body colour pattern. Investigation of the chorionic structure of the new species showed similar details to those of other species within this species complex. The phylogeny also placed this species into a monophyletic group with C.nigra (Uéno, 1928), C.elongatula (McLachlan, 1875) and C.fusca Kang & Yang, 1995.