The present study was carried out to explore fish species in the Dudhkoshi River of eastern Nepal. The study covered two seasons, including the post-monsoon of 2020 and the pre-monsoon of 2021. Ten permanent sampling sites were fixed to collect fish and measure water quality parameters. Fishes were collected using cast nets and gill nets with the help of local fishermen. Water quality parameters were measured using water-quality test kits and applying titration methods. A total of 21 fish species belonging to 3 orders, 6 families and 12 genera were recorded. Among them, the least concern (LC) was represented by 17 species, DD (2), VU (1), and NT (1). Higher fish diversity was associated with pre-monsoon than post-monsoon, and Cypriniformes was the most dominant order with 16 species, representing 76.19% of the total fishes collected. The dominant family, Cyprinidae, accounted for 11 species (52.38%) of the total, followed by Botiidae and Sisoridae, each with three species (14.28%), while Psilorhynchidae had two species (9.52%). Each of the Ailidae and Channidae accounted for a single species (4.76%). Dissolved oxygen (DO) was at optimum levels, ranging from 8.4 mg/l to 12.2 mg/l. Ranges of other parameters, including water temperature (12.0 -18.5⁰C), conductivity (41.6-82.66μs/ cm), total acidity (3.1- 4.0mg/ l), free CO₂, (3.0- 4.0mg/ l), hardness (30.3- 35.6mg/ l), alkalinity (16.2- 19.5mg/ l), salinity (19.3- 39.0ppm) and TDS (27- 56ppm) were determined. The redundancy analysis (RDA) was used to investigate the association between species and water quality parameters. The two axes of RDA explained 66% variation such that RDA₁ was accounting for 51%, and RDA₂ was accounting for 15%. A strong association was observed between DO and species such as Schizothorax. richardsonii and S. progastus. Similarly, Labeo dyocheilus, Barilius bendelisis, Psilorhynchus pseudecheneis and Botia lohachata showed high kinship towards conductivity, temperature and salinity.