Natural hazard triggering technological disasters (Natech) pose a major risk to chemical industries. Natural disasters, such as earthquakes, floods, and tsunamis, may cause massive fires, explosions, and toxic chemical release into the environment. Thus, Natech risks have recently attracted considerable research attention. Although Natech-related case studies are seldom reported in the literature, the results obtained thereof facilitate concrete risk assessments and countermeasure implementation against natural hazards. This article presents a case study on tsunami risk assessment performed by Asahi Kasei Corporation, Japan, based on four steps: (1) review of Natech events caused by the 2011 Great East Japan Earthquake, (2) hazard identification at the plant, (3) risk assessment considering human casualties, equipment loss, environment, and business continuity, and (4) evaluation and implementation of appropriate countermeasures. The risks posed by tsunamis were identified and analyzed. The inundation heights of Level-1 and Level-2 tsunamis were estimated to be 1 and 10 m, respectively. A tsunami evacuation tower and tidal wall were built as appropriate countermeasures to prevent or mitigate fatalities, loss of containment, and business interruption. The results obtained in this study facilitate adequate preparedness of chemical plants against future tsunami occurrences.