A hybrid plate-fin heat exchanger for exchanging heat between a first fluid and a second fluid is disclosed. The hybrid plate-fin heat exchanger comprises a plurality of plates, each of which comprises channels for conveying the first fluid. Fins are brazed onto each plate, wherein the fins define a plurality of flow channels for the second fluid. The plates are joined to one another via friction-stir welding in such a way that the brazed regions are fluidically isolated from the first fluid during operation. As a result, the heat exchanger is suitable for use in applications that use a first fluid, such as seawater or geothermal fluid, which is corrosive for the brazed regions.