A new species of tenthredinid sawfly, Mesoneura tematinensis Roller, sp. nov., was discovered in the Tematinske kopce Mountains in the Western Carpathians in Slovakia. Adults of both sexes and larvae of different stages are described and illustrated. Based on morphology and DNA barcoding, the new species is closely related to Mesoneura opaca (Fabricius), a widespread oak sawfly in Europe, with which it occurs in the same locality and shares a common host plant. Larvae of the new species are part of a rich assemblage of a total of 13 Symphyta species that feed on leaves of the pubescent oak Quercus pubescens in a thermophilic supra-Mediterranean forest. A key to the European species of Mesoneura Hartig is provided.