Provided are an exhaust gas treatment catalyst capable of reducing ammonia leakage rate while keeping a sufficient NOx removal efficiency, said catalyst comprising a coating layer and a catalyst base material, wherein said coating layer has a decreased thickness relative to that of a catalyst base material; and an exhaust gas treatment system using the same. In the exhaust gas treatment catalyst for catalytically removing nitrogen oxides from an exhaust gas by using ammonia as a reducing agent and simultaneously decomposing and removing unreacted ammonia, a coating layer comprising platinum supported on titania is formed on a surface of a porous catalyst base material comprising titania and at least one compound selected from oxides of vanadium (V), oxides of tungsten (W) and oxides of molybdenum (Mo).