A device for controlling the temperature of a swimming pool is disclosed. The device measures the temperature of the water as well as other environmental conditions (such as air temperature). The device controls a circulation pump, which circulates water throughout the pool. The pool water exchanges heat with the surfaces of the water (including the side walls, ground and surface air) when a heat differential exists between the water and the surfaces. When the water is circulated throughout the pool it increases the volume of water that exchanges heat with the surfaces, thus cooling or heating the water depending on the temperature of the surfaces. A target temperature is entered into the device and using this target temperature the device automatically controls the pump in order to take advantage of the heat exchange between the water and the water's surfaces to alter the water temperature in the direction of the target temperature. Additionally a heating element may be used to supplement the heating operation of the circulation pump.