In developing countries, the intensification of urban processes affects the correct management of the territories. Specific natural hazards such as landslides, floods or soil sealing are influencing several urban settlements worldwide. One example can be found in the oldest city of the world, i.e., Hamadan City, where floods are associated with human and economic losses and the destruction of several ancient sites. However, solutions to be implemented to reduce the risks in such a city are still scarce. Therefore, this research aims to evaluate roof rainwater harvesting (RRWH) in reducing the urban floods in Hamadan City. To achieve this goal, three different scenarios are tested, i.e., simulation of the rainfall-runoff process through the Hamadan City (1) without any RRWH, (2) considering that the harvested rainfall is used for household consumes, and (3) considering that the RRWH is used for garden irrigation. The results showed that scenario 2 (using harvested rainfall for household consumption) presented reductions of 30.4% and 28.52% in the inflow volume compared to scenario 1 respectively for 2-year and 100-year return periods, whereas these reductions were 38% and 31.9% for scenario 3. Besides, a remarkable difference was noted between scenarios 2 and 3. In scenario 3, we found that the current drainage network has more capability to transfer surface runoff through Hamadan City, which would help reduce the risk of floods in the city. Therefore, it is concluded that the policymakers and land managers should consider these possibilities, and awareness of the urban inhabitants must be enhanced to reduce the risks and promote sustainability.