We investigated damage to mangroves from the 2004 Indian Ocean tsunami at Pakarang Cape and Namkem, Thailand. Visual inspection of remotely sensed images revealed that 12 and 20 ha of mangrove forests were damaged, respectively, at Pakarang Cape and Namkem. Field surveys revealed that mangrove trees were destroyed particularly around the river mouths and channels. Numerical simulation indicated that the flow concentrated at the river mouth and inundated mangrove forests through river channels. We concluded that the tsunami flow pattern was largely affected by local river mouth and channel topography. Damage to mangroves might be severe in such areas.