An exhaust system and a gas turbine, in which the exhaust system includes an exhaust casing; an outer diffuser supported inside the exhaust casing; an inner diffuser disposed inside the outer diffuser to form an exhaust gas flow passage between the inner diffuser and the outer diffuser; a strut cover coupled at a first end to the outer diffuser and coupled at a second end to the inner diffuser; and a second cooling air introduction port provided in the exhaust casing, further on the downstream side of the exhaust gas flow passage than the strut cover. Furthermore, a non-perforated partition member having an annular shape is arranged so as to cover the outer side of the outer diffuser and is supported by the exhaust casing to form a cooling air flow passage.