This paper presents a flash cooling desalination system to reduce thermal pollution and also to produce freshwater using the heat rejected by the process plant into the environment. The prototype plant was erected in an existing coal-based thermal power plant at North Chennai, India. It consists of an air-tight barometric sealed flash cooler, positioned at a level at least 10.13 m above the ground level, for the feed seawater to flow under the effect of gravity and to maintain a low pressure. The prototype plant was tested by using a small fraction of the available flows without using any mechanical energy such as motive steam from the power plant. A freshwater production rate of 0.49% of the feed seawater is obtained from the available thermal gradient of 8.5 °C from the condenser reject heat of the power plant, and then the waste water is discharged at near intake concentration of salinity into the sea. The temperature of hot feed seawater is also reduced by 3 °C. The results are used to provide an outline technical specification for larger capacity desalination plant to meet the growing need for freshwater. This is an environment friendly desalination process and consumes no chemicals as it operates at near ambient temperature. This can be effectively utilized for the generation of freshwater, besides protecting the marine ecosystem along the shore, and reducing the load on the cooling tower or eliminating the need for it completely. Copyright © 2012 John Wiley & Sons, Ltd.