An emerging soil-borne disease resembling Phytophthora stem rot was observed on mung bean plants grown in Anhui, China. To identify the causal agent, diseased plants and soil samples from 13 fields were collected to isolate the pathogen. Twenty-two Phytophthora isolates were recovered from the samples and conducted for detailed identification. Based on morphological and molecular characterizations, all the isolates were consistently identified as Phytophthora vignae. Phylogenetic analysis using eight nuclear loci sequences of the internal transcribed spacer (ITS) region, rRNA gene large subunit (LSU), a partial sequence of the beta-tubulin (β-tubulin) gene, the translation elongation factor 1 alpha (EF1-α), the 60S ribosomal protein L10 (60SL10), enolase (Enl) gene, the heat shock protein 90 (HSP90), and the triose phosphate isomerase/glyceraldehyde-3-phosphate dehydrogenase (TigA) and a mitochondrial locus cytochrome c oxidase subunit I (cox1) revealed that the mung bean isolates group in the same clade as P. vignae and its two formae speciales, P. vignae f. sp. adzukicola and P. vignae f. sp. vignae. A host specificity test showed that the mung bean isolates of P. vignae are pathogenic towards mung bean with a much stronger virulence and towards adzuki bean with a relatively weak virulence, but non-pathogenic to the other tested legume crops, soybean, cowpea, pea, common bean, faba bean, and chickpea. The host range of mung bean isolates significantly differs from that of the P. vignae f. sp. adzukicola and f. sp. vignae, based on our results and the previous studies. Thus, the pathogen causing Phytophthora stem rot of mung bean is proposed as a new forma specialis of P. vignae, designated as P. vignae f. sp. mungcola.