We report hirudiniasis caused by the leechHemiclepsis marginata asiaticaMoore, 1924 in albino red-bellied pacu (pirapitinga)Piaractus brachypomus(Cuvier, 1818), constituting the first documentation of a freshwater fish species being affected in India. The outbreak occurred in a tank of an aquarium-fish retailer; infested fish appeared asphyxiated, unable to swim or swimming upside down, with cloudy eyes and body with thick mucus secretion. The prevalence and mortality was 100%, with a mean intensity of 81 leeches per fish. The histopathology of the morbid fish revealed degenerative necrosis, eosinophilic infiltration in the muscle tissue and haemorrhages in the fin membrane. The leech mitochondrial 18S rDNA and 12S rDNA genes were characterised and submitted to GenBank under accession numbers MN380443 (18S) and MK733282 (12S). A maximum likelihood tree was constructed using 12S rDNA gene sequences to demonstrate the phylogenetic position ofHemiclepsis marginata asiaticaamong its congeners.