The Old Brahmaputra Floodplain of Bangladesh frequently faces the severity of floods during the monsoon because of its geographical location through the low-lying land with dynamic morphological behavior due to an immense load of upstream discharges. This research focuses on the significant parts of the Old Brahmaputra Floodplain (Mymensingh, Jamalpur, and Sherpur Sadar Upazila) because of the destructive flooding history. This study aimed to generate a comprehensive micro-scaled flood hazard map using the HEC-RAS (Hydrologic Engineering Center's River Analysis System) coupled with 1D-2D numerical modeling approaches in a quantitative way for different time intervals (10, 20, 50 & 100 years) based on the discharge hydrograph generated after the frequency analysis using the HEC-SSP (Hydrologic Engineering Center's Statistical Software Package). Log-Pearson Type III distribution has been used for frequency analysis to estimate the peak discharge for the different return periods. After performing the Unsteady Flow simulation for the model, calibration and validation have been performed for the 1D model concerning the observed water level of Mymensingh station (SW228.5). Finally, flood hazard maps were produced performing Unsteady Flow simulation of the coupled 1D-2D model for different return periods (5, 10, 20, 50, and 100 years) showing flood extent, water depth, velocity, maximum arrival time, and duration for each return period which is one of the most widely used ways to identify vulnerable areas and exposures to floods so that the authorities can take the necessary precautions for those exposures to minimize the losses and flood risk. Therefore, this study will help generate a more comprehensive flood hazard map of the Old Brahmaputra River to strengthen the existing flood risk management policies and protection measures.