Monocillium gamsii sp. nov. (Ascomycota, Hypocreales, Niessliaceae) isolated from eggs of the cereal cyst nematode Heterodera filipjevi is described and illustrated based on morphological and molecular phylogenetic evidence. The new taxon discovered in wheat fields in Turkey destructively parasitises nematode eggs. The infected eggs were readily colonised by the fungus, which produced microsclerotia. The fungus could be grown on artificial media and the parasitism of M. gamsii towards H. filipjevi was reproducible in vitro. Hyphae penetrating the nematode eggs entirely colonised the embryo, developed into multicellular chlamydospore and dictyochlamydospore-like structures eventually forming microsclerotia. Molecular and morphological differences and similarities between M. gamsii and its phylogenetically related species are discussed. Monocillium bulbillosum was found to be closely related to the new species. The pathogenicity of M. bulbillosum against H. filipjevi was also assayed in vitro because of its sister group relationship to M. gamsii revealing that this species was also capable of colonising eggs of H. filipjevi.