The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on 11 March and generated strong shaking and a huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom. To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea environment off Sanriku, we conducted shipboard surveys up to ~2000 dbar during 2011–2018 and long-term monitoring of the seafloor on the continental slope using a deep-sea station (~1000 dbar) off Otsuchi Bay during 2014–2018. The turbidity calculated from light transmissometer anomaly data for the bottom layer deeper than 500 dbar on the continental slope was ~6% and higher than the values usually observed there. This high turbidity during 2011–2018 was caused by sporadic sediment resuspension induced by frequent aftershocks in the area off Sanriku. We found that dissolved oxygen concentrations between 1000 dbar and 1500 dbar dropped significantly by about 10% after the earthquake, whereas nutrients and dissolved inorganic carbon showed no significant difference and displayed a wide variation. The correlation between light transmissometer anomaly values and chemical parameters at 1000–1500 dbar suggests that turbidity increased the concentrations of phosphate, silicate, dissolved inorganic carbon, and methane, and decreased dissolved oxygen and nitrate. The deep-sea chemical environment was changed by remineralization resulting from the respiration and denitrification of microbial communities after the earthquake. The environmental change in the deep sea was maintained by sediment resuspension caused by continued aftershocks and was likely caused by variations in dissolved inorganic carbon and nutrients on the slope after the mainshock and aftershocks. The concentration and carbon isotope ratio of methane between 1000 and 2000 dbar near the hypocenter shows two peaks with high light transmissometer anomaly values near the isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). These sources of chemical input contributed by sediment resuspension induced by the mainshock and aftershocks might have been derived from shallow sediment on the continental slope and deep sub-seafloor reservoirs.