Leptosillia is a fungal genus in the monogeneric family of Leptosilliaceae. The genus was circumscribed by Austrian mycologist Franz Xaver Rudolf von Höhnel in 1928. As the genus name suggests, Leptosillia was considered to be closely related to the diaporthalean genus of Sillia (in the Stilbosporaceae family, Diaporthales order).
The genus was monotypic for a long time, containing only the type species, Leptosillia notha. Until molecular phylogenetic analysis published in 2019 showed that the genus belongs to the order Xylariales, and that the genus Cresporhaphis should be included in Leptosillia. These analyses placed Leptosillia as a sister taxon to family Delonicicolaceae, and so a new family, Leptosilliaceae, was circumscribed to contain it. Both families were then placed in order Delonicicolales, which forms a highly supported clade (100% ML). This was accepted by Wijayawardene et al. 2020.