A waste heat recovery device for a marine vessel is disclosed. According to the embodiments of the present invention, the waste heat recovery device for a marine vessel includes: a heat exchanger, which recovers heat from the exhaust fumes discharged from the engine, to heat a first refrigerant under uniform pressure; a turbine which is driven by adiabatically expanding the first refrigerant heated under uniform pressure; a condenser which condenses the adiabatically expanded first refrigerant; and a heat exchanging pump which compresses the condensed first refrigerant so as to re-circulate the compressed first refrigerant to the heat exchanger.