Freshwater fish are on the brink of extinction due to the degradation of habitat. The present research focused on the estimation of the environmental flow requirement of significant fish species in the Nambul River, Northeast India. Nambul is an important river that drains directly into Loktak Lake. Loktak Lake is an internationally important Ramsar-designated wetland. The environmental flow requirements of the significant fish, namely Bangana dero and Wallago attu, were estimated using the physical habitat simulation model (PHABSIM). The habitat suitability curves (HSCs) were employed in PHABSIM along with the instream habitat data of the cross-sections for estimating the weighted usable areas (WUAs) of the two fish species. Based on the WUA-discharge curve, an optimum environmental flow requirement of  50 cumecs of discharge is recommended for the fish habitats. The estimation of minimum environmental flow requirements can be considered as an essential step toward conservation and sustainability.