The apparatus for water treatment using in-situ activation of a manganese dioxide catalyst includes: a reaction bath configured to give a space where aqueous organic contaminants are removed by means of reaction with permanganate (MnO4−) generated by electrochemical oxidation of manganese oxide (MnO2); a plurality of manganese dioxide catalysts provided at the reaction bath and electrochemically oxidized into permanganate (MnO4−) by a voltage applied thereto; and a power supply device configured to apply power to the manganese dioxide catalyst so that the manganese dioxide (MnO2) is electrochemically oxidized into permanganate (MnO4−).