Underwater acoustic reflection characteristics for a thin stiffened aluminum plate were investigated. The thin stiffening rib extends parallel to the edge across the entire width of the plate. A plain plate without the stiffening rib was used as a control. The targets were sequentially suspended in water during the experiments. The stiffening rib on the experimental target was positioned in the acoustic shadow of the plate to avoid specular returns from its edges. Internally guided longitudinal stress waves were detected in both targets for a critical angle of incidence of 15.8°, at a frequency of 60 kHz and a 30‐μsec pulse width. Based on ray theory and the assumption that these stress waves were reflected at the reinforcing rib, theoretical travel times, and pulse‐reflection periods were calculated. Good agreement between theoretical and experimental results was achieved. Pulse decay rates in the form of logarithmic decrement of internally guided longitudinal stress waves are shown to be a significant clue to the presence and location of a reinforcing rib in thin plates.