A nectrioid fungus forming a pinkish colony with mainly solitary phialides producing ellipsoid, aseptate conidia in mucoid packets was isolated from Dirinaria applanata. Our taxonomic study based on morphology and phylogenetic analysis using ITS rDNA sequences revealed that the isolates represented a member of the genus Cylindromonium. Based on further morphological examination, nucleotide sequence comparison, and phylogenetic analysis based on LSU rDNA, tef1, and rpb2 in addition to the phylogenetic analysis using the ITS rDNA sequences, the fungus from Dirinaria represents a new species, which is described here as Cylindromonium dirinariae sp. nov. Furthermore, inoculation experiments revealed that this species can also produce perithecia when inoculated on the host lichen in laboratory environments. Citation: Ohmaki A, Okane I, Crous PW, Verkley GJM (2023). Cylindromonium dirinariae sp. nov. (Ascomycota, Hypocreales), a new nectrioid lichenicolous species on Dirinaria applanata in Japan. Fungal Systematics and Evolution 11: 1-10. doi: 10.3114/fuse.2023.11.01.