The Old Brahmaputra River is a vital water resource in Bangladesh, which faces numerous difficulties because of natural variability, human activity, and climate change. This study uses the Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) to evaluate the environmental flow requirements (EFR) of the Old Brahmaputra River under historical and future climate change scenarios. Three hydrological techniques are used in the study to assess the EFR for both low-flow and high-flow seasons: the Tennant Method, the Flow Duration Curve (FDC) Method, and the Constant Yield Method (CYM). Using CMIP6 climate models (ACCESS-ESM1-5, BCC-CSM2-MR, and MRI-ESM2-0) under the SSP5-8.5 scenario, future projections were made for the 2030s, 2050s, and 2080s based on historical data from 1994 to 2024. The findings show notable seasonal fluctuations in flow, with the river suffering from severe ecological degradation during the low-flow period, especially in March. The FDC and CYM methods offer more precise, month-by-month estimates, whereas the Tennant Method recommends an EFR of 62.22 cumecs for the low-flow season. The ACCESS-ESM1-5 model predicts considerable increases in monsoon flows in the future, which could result in more frequent and severe flooding. Future projections show growing flow variability. On the other hand, the MRI-ESM2-0 model projects lower future flows, indicating potential water scarcity. In order to meet the challenges brought on by climate change, the study shows the importance of adaptive water resource management strategies, such as preserving minimum flows during the low-flow season. The results highlight the necessity of integrated water resource management to maintain the Old Brahmaputra River's ecological sustainability and sustain the livelihoods of the communities that depend on it.