<p>This study assesses the impact of climate change on water availability for agricultural and domestic use in the Indian state of Sikkim, which lies within the Teesta basin in the Eastern Himalaya. An integrated modeling approach combining a hydrological model (HydroRAM) and a water allocation model (MODSIM) was employed for this assessment. HydroRAM was compared with the SWAT model and showed improved performance, as indicated by NSE/R² of 0.84/0.87 (calibration) and 0.89/0.88 (validation), compared to SWAT’s 0.78/0.84 and 0.68/0.86, respectively. Future water availability under the impact of climate change was analyzed using the ensemble of five CMIP6 climate models under two Shared Socioeconomic Pathways (SSPs), SSP245 and SSP585, for the period 2027–2060, and the results were compared with the model’s historical baseline period (1990–2014). For a detailed assessment, the entire state was divided into four zones: North, East, South, and West. The findings indicate that agricultural and domestic water shortages will be most severe in South Sikkim. Agricultural water shortages in South Sikkim are projected to increase by 43.26% under SSP245 and 28.66% under SSP585. On the other hand, domestic water shortages are expected to rise by 102.71% (SSP245) and 84.29% (SSP585). Additionally, under SSP245 and SSP585, domestic water shortages in South Sikkim are projected to be 57.30% and 53.26% higher, respectively, than those expected for agricultural water shortages. Among the remaining zones, East Sikkim is projected to experience the second-highest shortages. The findings of this study highlight the importance of strategic planning and management in the Teesta River basin, providing a strong basis for informed decision-making to balance competing water demands and optimize resource allocation.</p>