Euphorbia hirta is an important medicinal herb that can be found on grassland, roadsides, and field pathways in tropical regions of world. In late October and early November 2016, powdery mildew was observed on E. hirta growing along roadsides in Fujian Agriculture and Forestry University (26°5’16”N, 119°14’6”E), Fuzhou, China. Disease incidence approached 100%, with fungal colonies present on stems and both sides of the leaves. At the initial stages of infection, colonies were circular to irregular and later covered the entire leaf surface. Hyphae were septated, branched, flexuous to straight, up to 8 μm wide with indistinct to slightly nipple-shaped appressoria. Conidiophores were unbranched, straight, 90 to 260 × 10 to 13 μm in size. Foot-cells of the conidiophores were cylindrical, 40 to 80 μm long, with slight constriction at basal septa, and followed by 1 to 3 short cells. Conidia were hyaline, formed in chains of 2 to 6 cells with distinct fibrosin bodies visible in their cytoplasm, ellipsoid-ovoid to barrel-shaped, about 30 to 45 × 14 to 20 μm in size, with a length to width ratio of mostly 1.5 to 2.0. Chasmothecia were absent. The rDNA internal transcribed spacer (ITS) region was amplified from pools of conidia on infected leaves by using primers ITS1/ITS4 (†) and directly sequenced. BLASTn analysis of the 569 bp (KY388504) amplicon revealed 100% sequence identity with respective rDNA sequences of Podosphaera xanthii isolates from Asteraceae (Bidens pilosa [KM260740], Youngia denticulata [AB040351]), Euphorbiaceae (Euphorbia thymifolia [KM260731]), Solanaceae (Physalis angulata [KM260744]), and Verbenaceae (Verbena brasiliensis [KJ472787]). Phylogenetic analysis also indicated clustering of E. hirta fungal isolate with P. xanthii strains. Based on the morphological characteristics and ITS sequences, the fungal species was identified as P. xanthii (Castagne) U. Braun & Shiskoff (†). Pathogenicity of the fungal isolate was confirmed by gently pressing diseased leaves onto the leaves of three healthy E. hirta plants, resulting in the formation of white powdery mildew colonies after 7 days at 20°C and 80% relative humidity in a greenhouse. Fungal colonies on the inoculated plants were morphologically identical to the collected fungal specimen, whereas control plants of E. hirta developed no symptoms of powdery mildew. Until now, P. euphorbiae-hirtae was documented on E. hirta, E. tithymaloides, and A. australis (†;†). However, recently Golovinomyces orontii has also been reported on E. hirta in India (†). E. hirta is an important medicinal plant in China and present information will be helpful in understanding the range of pathogen that can infect this plant. To our knowledge, this is the first report of P. xanthii on E. hirta.