Dematiopleospora mariae, isolated from Ononis spinosa collected in Forlí-Cesena Province in Italy, is introduced as a new ascomycete genus and species based on morphology and combined LSU and SSU sequence analyses. Phylogenetic analysis based on maximum parsimony (MP), maximum likelihood (ML) and Mr Bayes all support Dematiopleospora as being a distinct genus within the Phaeosphaeriaceae. Dematiopleospora is distinguished from other genera in this family in having ascospores whose central cells have longitudinal septa with light end cells, ascomata with a thick peridium and necks comprising short, light brown setae. Phylogenetic analysis also separates Dematiopleospora from other genera in the Phaeosphaeriaceae. Dematiopleospora forms a sister group with Entodesmium and Chaetosphaeronema, but these three genera are clearly separated in the molecular analysis with relatively high bootstrap support (89% and 99% respectively). The new genus is compared with similar genera of Phaeosphaeriaceae and a comprehensive description, and micrographs are provided.