Climate change can alter water availability and hydro-energy production, and therefore, affects energy security as well as nexused relationship between water and energy. This study optimized the operational rule curve for hydro-energy generation in the context of climate change impact in Tamor storage project located in the Eastern Nepal. The physical data, height-area-volume data, rule curve and hydrological data were used in the HEC-ResSim model for baseline (historical, 1989–2009) and future (2021–2095) periods. Then, reservoir operational rule curve was evaluated under various scenarios. The energy production potential for the base case was assessed using five operational rule curves (RCs), among them, RC-5 produced the highest annual and dry energy. With consideration of climate change, three RCs maximized dry energy and ensured reservoir level is at their lowest operational level by the end of dry season. Among the three, RC-5 produced most of the energy both annually and during the dry season. Hence, RC-5 is recommended with considering of climate change as well. For consideration of nexus, rule is made such that 2 m3/s of water is release to the downstream throughout the year and RC-5 as adopted for energy generation in the reservoir. These findings are expected to help optimize energy production from a storage reservoir in consideration of nexused relationship as well as climate change impacts.