Because property damage and loss of life owing to floods have increased in many countries in recent years, accurate assessment of flood risk is urgently needed for effective flood management. Improving the assessment of flood risk requires considering the impact of dam operation for flood control because dams and reservoirs play important roles in the assessment of flood hazards and associated damage. However, the impact of dam operation on flood hazards and associated damage was not considered well in previous assessments of flood risk. This paper focused on the quantitative assessment of flood hazard and risk, including the effectiveness of dam operation for flood prevention. The grid-based approach integrating the following was applied for analysis: (i) a hydrological–hydraulic model, (ii) a method considering the dam operation for flood control, and (iii) an assessment of flood damage. To assess the risk, flood characteristics were computed using the rainfall–runoff–inundation hydrological–hydraulic model, and flood damage was estimated by integrating the flood characteristics, flood damage curves, exposure characteristics, and property values. The risk was assessed by focusing on flood damage to residential buildings and assets and agricultural sectors for the largest recent floods and flood events with different return periods. This study considered the Bago River Basin of Myanmar. Results show that the dam operation for flood control in the study area reduces the flood inundation area by approximately 10% and flood damage to buildings, assets, and agriculture by approximately 40%, 60%, and 10%, respectively.