An outboard marine propulsion system with a closed loop lower unit heat exchanger is described. The closed loop lower unit heat exchanger provides improved cooling to outboard marine propulsion units and increases engine life compared with traditional open loop cooling systems, particularly in harsh environments such as salt water, brackish water, swamp, shallow or sediment and debris rich water. The closed loop lower unit heat exchanger also provides improved performance to the outboard marine propulsion system when mounted to an appropriate portion of the lower unit of the outboard marine engine such as the anti-cavitation plate.