<p>Rondônia is one of the Amazonian states with a small number of reports of the family Leptophlebiidae, with only five species. The lack of data is particularly relevant because the group is ignored in the decision-making process (land-use, areas of conservation and climatic mitigation). The aim of this study is to provide data on the biodiversity of Leptophlebiidae from Rondônia by compiling old and new records and to describe a new species <i>Thraulodes inopinatus</i> sp. n. The results added 11 new records for the state, increasing the number of species from five to 16. Rondônia has now become the third state with the most records of Leptophlebiidae species in the northern region (Brazil). The newly discovered <i>Thraulodes inopinatus</i> sp. n. has hyaline membrane fore and hindwings, differentiating it from <i>T. marianoi</i> and <i>T. basimaculatus.</i> The median projection of the styliger plate has a rounded shape with a dorsal curvature, the penis lobe is wide medially and taper towards the apex, and the telopenis groove opens ventro-laterally, differentiating the new species from <i>T. niger</i>, <i>T. nigrabdominalis</i>, <i>T. nigripes</i> and <i>T. nigrotibialis</i>.</p>