This research performed a case study to develop an integrated Flood Risk Index (FRI) considering hydrologic, socio-economic, and ecological components to assess watershed-based flood hazards and vulnerability in the Korean Han River Basin, where flood disasters frequently occur. We used fifteen indicators, which were standardized, weighted, and classified, to create an FRI map. Five categories of flood risk were distinguished: “very high”, “high”, “medium”, “low”, and “very low”. The results in ternary diagrams, illustrating the relative importance of the three components show that urban areas in the Korean Han River Basin have experienced a decrease of 1.0% in terms of the hydrologic component. However, the socio-economic and ecological components have increased by 3.2% and 4.4%, respectively. In rural areas, an increase of 4.8% was found in the hydrologic component alongside decreases in the socio-economic (1.6%) and ecological (2.2%) factors. In addition, the FRI map shows that 20.02% of the total area of the Han River Basin is classified as having “very high” or “high” flood risk; the size of this high-risk zone has increased significantly, by 7.12, since 1990. Despite the relatively small sample size and lack of data, the results presented here provide a quantified assessment of Han River flood risk combined with an integrated view of risk assessment and management for stakeholders and decision-makers.