On 11 March 2011, a massive tsunami generated by a mega-earthquake with a moment magnitude of 9.0 hit a wide area of Pacific coast of northeast Japan. We observed and analyzed the effects of the earthquake and tsunami event on populations of the abalone Haliotis discus hannai and sea urchin Strongylocentrotus nudus at Tomarihama in Miyagi, where we have carried out regular surveys since January 2008. Before the event, algal forests dominated by the brown macroalga Eisenia bicyclis had developed in the survey area shallower than 5 m in depth, where adult abalone >50 mm in shell length (SL) inhabited. Juvenile abalone <20 mm SL and juvenile and adult urchins inhabited the deeper area dominated by crustose coralline algae (CCA). After the event, although no apparent decrease was observed in the brown macroalgal population, the mean density of adult abalone >50 mm SL, mainly inhabiting the algal forests, was reduced by more than half. The impact of the tsunami was more profound in the CCA area than in the macroalgal forest. Juvenile abalone and urchins largely decreased to 14 and 5% of the densities just before the event, respectively. The distribution pattern of juvenile abalone and urchins could be a cause of the marked decrease, because most of these animals inhabited the CCA area where the disturbance by the massive water movement was not reduced by the effects of the macroalgal forest.