Mayflies collected in the central mountain range of New Guinea (Highland Papua Province) revealed a new genus of Baetidae, Siphliops gen. nov. The larvae present a unique combination of characters, particularly: abdominal tergum IX with acute extension of the posterolateral corners; carina-like elevation on the frons; labium with glossae and paraglossae spread, and glossae much shorter than paraglossae. The new genus belongs to the tribe Labiobaetini. A new species, Siphliops extenso gen. nov. et sp. nov. is described and illustrated based on larva and subimago. Remarkably, it is known from a single locality only, despite the large amount of sampling done in New Guinea in the past years. COI sequences were obtained from three specimens. Morphological similarities and the relationship of the new genus to other genera of Baetidae are discussed.