A catalytic reactor according to an embodiment includes a catalytic unit including at least one catalyst having a honeycomb structure in which a plurality of passages extending in an axial direction are formed, a reactor housing accommodating the catalytic unit, and a seal plate sealing between an outer periphery of the catalytic unit and an inner periphery of the reactor housing. The seal plate seals between the outer periphery of the catalytic unit and the inner periphery of the reactor housing at an upstream end portion of the catalytic unit with respect to a flow of a fluid flowing in the reactor housing.