Enormous flooding in the lower part of the Yom and Nan River Basins and the Chao Phraya River Basin in the second half of 2011 resulted in tremendous loss and adversely impacted on the livelihood, society, and economy of Thailand. The development of reliable and efficient prediction tools is important for prevent similar occurrences in the future. Consequently, this study thus aimed to develop a regional hydrological model including the inundation effect. A flow routing model that included as a part of the regional hydrological model was modified based on the concept of a diffusive tank model to include the inundation effect. Overbank flow was estimated by using a broad crested weir equation. The inundation model had ten parameters that provided a good fit for observed and simulated discharge in 2011 at the C.2 Station with a Nash-Sutcliffe efficiency of 0.91.