The present invention is a flow deflector suitable for a heat exchanger of the type consisting of at least one core made up of tubes forming a bundle arranged inside a shell and at least one baffle. The deflector according to the invention is an easily manufactured part independent from the construction of the bundle of tubes and of the baffle which allows modifying the coolant fluid or liquid flow path with greater freedom than that achieved by combining internal openings in the baffle or baffles. Another object of this invention is the heat exchanger obtained using the deflector for optimising coolant liquid flow path. The application of the invention in heat exchangers for EGR (Exhaust Gas Recirculation) systems is of special interest.