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The 2004 Indian Ocean tsunami deposited a sheet of sand with surficial bedforms at the Andaman coast of Thailand. Here we show the recognition of bedforms and the key internal sedimentary structures as criteria of the tsunami supercritical flow condition. The presence of well‐preserved capping bedforms implied a dominant tsunami inflow. Sets of internal sedimentary structures including parallel lamination, seaward and landward inclined‐laminations, and downstream dipping laminae indicated antidune structures that were generated by a supercritical flow current in a depositional stage during the inflow. A set of seaward dipping cross‐laminations containing sand with mud drape on the surface of one depositional layer are a unique indication of an outflow structure. A majority of deposits show normal grading, but in some areas, localized reverse grading was also observed. The recognition of these capping bedforms and determination of the internal sedimentary structures provides new key criteria to help derive a better understanding of tsunami flow conditions.