Achlya bisexualis is a notorious oomycete pathogen with the potential to cause emerging disease in fish farms. In this study, we report the first isolation of A. bisexualis from captive-reared golden mahseer Tor putitora, an Endangered fish species. The infected fish showed a cotton-like growth of mycelia at the site of infection. The mycelium when cultured on potato dextrose agar produced radially growing white hyphae. The hyphae were non-septate, and some of them carried matured zoosporangium with dense granular cytoplasmic contents. Spherical gemmae with stout stalks were also observed. All the isolates had 100% identity in internal transcribed spacer (ITS)-rDNA sequence and showed highest similarity to that of A. bisexualis. In molecular phylogeny, all the isolates formed a monophyletic group with A. bisexualis which was supported by a bootstrap value of 99%. Based on the molecular and morphological findings, all the isolates were confirmed as A. bisexualis. Further, the anti-oomycete effect of boric acid, a known antifungal agent, against the isolate was evaluated. The minimum inhibitory concentration and minimum fungicidal concentration were found to be 1.25 and >2.5 g l-1, respectively. Isolation of A. bisexualis from a new fish species indicates its possible occurrence in other unreported hosts. Considering its wide infectivity and the potential to cause disease in farmed fishes, its probable prevalence in a new environment and host needs to be closely monitored to prevent the spread of infection, if any, by adopting suitable control measures.