The genus Cymbalcloeon gen. nov. (Ephemeroptera: Baetidae) is established for a new species Cymbalcloeon sartorii sp. nov. from Thailand, based on larval stage. This genus is unique among all of the Baetidae by the presence of three pairs of single gills on segments V–VII, ventrally oriented to cover the sterna VI–IX. Cymbalcloeon sartorii gen. nov. et sp. nov. presents unique or rare morphological characters such as a deeply concave margin between the prostheca and mola, without setae; a very large subtriangular process of the left mandible; a maxillary palp segment II with scarce and very long setae; almost completely fused labial palp segments II and III with numerous very long setae; elongate tarsal claw with two rows of teeth; a shagreen surface of the terga and paraproct; and a very reduced body size. The gills of the new genus move very quickly during respiration and present a near-synchronous protraction. Due to the very derived larval morphological character and the unknown imaginal stage, the exact phylogenetic position of the genus remains unclear; it most certainly belongs to the concept of Anteropatellata and is possibly closely related to the genus Baetopus.

Megabranchiellagen. nov. (Ephemeroptera: Baetidae) is established as a new baetid mayfly genus from northern Thailand. Two new species, Megabranchiella scutulatasp. nov. and Megabranchiella longusasp. nov., are described. This genus is distinguished from other Baetidae by abdominal segment I, bearing a pair of enlarged, ventrally oriented single gills, covering abdominal sternites II–V; other gills have normal size and are dorsolaterally oriented. The two new species Megabranchiella longusasp. nov. and Megabranchiella scutulatasp. nov. can be differentiated by the setation of femur dorsal margin and the shape of abdominal gill I. This mayfly genus was found in flowing water with cobble microhabitats in headwater streams of northern Thailand.

A new species of the genus Tenuibaetis Kang & Yang, 1994 is described from Thailand and the genus is reported for the first time from this country. Tenuibaetis panhai sp. nov. is easily distinguished from other known Tenuibaetis by the complete absence of hindwing pads. Molecular evidence based on COI confirmed the validity of the new species. Additional putative species of Tenuibaetis based on molecular evidence only are considered as Molecular Operational Taxonomic Units (MOTUs) without description. The morphological characters of the new species and its closely related species are discussed; a key to the Oriental species is provided.

The genus Procerobaetis Kaltenbach & Gattolliat, 2020 is reported for the first time from Thailand, and Procerobaetis totuspinosus sp. nov. is described as a new species based on larvae. It can be easily distinguished from other known Procerobaetis species by the presence of triangular spines at the posterior margin of tergites VI–IX. COI sequences were obtained from all known species. The genetic distances (Kimura 2-parameter) between the new species and the other species are between 20% and 23%. The morphological characters of the new species and its closely related species are discussed; larval key to all species of the genus Procerobaetis is also provided.

Platybaetis bishopi Müller-Liebenau, 1980 was originally described from Malaysia only at the larval stage. We provide the first description of the imaginal stage of P. bishopi based on materials from Thailand. The imago of this species can be separated from the known species by coloration of abdominal terga and coloration of wings. A new species, Platybaetis nayokensis sp. nov., is described based on male and female imagos and larvae from Thailand. The larva of this species is mainly distinguished by medium acute spines on the posterior margin of the abdominal terga and two apical setae on the glossa, which seem to be shorter than in other species. The imago can be separated by the abdominal color pattern. The larva of this genus is adapted to live on wet rocks projecting out of water; it prefers large stones near small waterfalls or areas between two large rocks in running water.

A newly discovered representative of Leptophlebiidae is described and illustrated from Thailand as Sangpradubina thailandica, gen. nov. and sp. nov., based on larvae and reared adults. This new species presents an interesting transitional position, being close to Choroterpes s.l. in the winged stages, but closer to Thraulus s.l. in the nymphal stage. Nymphs are differentiated from those of all genera of the Thraulus lineage by the combination of gill I consisting of only a single and slender lamella, gills II–VII being fimbriate only in the distal half, each mandible having a tuft of setae only in the middle of the lateral margin, each maxilla having a well-developed anterolateral projection, fore- and middle femora each having a row of long setae on the outer margin, as well as tarsal claws having 4 denticles at the base and 9 in the apical position. Adults of the new genus barely differ from any genera of the Choroterpes lineage. The egg chorionic structure of Sangpradubina also appears to be unusual in that KCT’s are concentrated on one pole.