Underwater workers, sport and military divers, are exposed to thermal stress since most of the waters of the world are below or above what is thermally neutral. Although divers wear insulation suits for passive thermal protection they are inadequate. Active heating is currently accomplished by resistive heating and open-flow tubes delivering hot water; however, these methods are problematic. The challenge of this project was to design, build and test an active diver thermal protection system (DTPS) to be used with wet suit insulation that is effective, user-friendly, reliable, and that could be used by a free-swimming diver. The DTPS has a minimum number of moving parts, is low maintenance, has no unsafe or toxic working fluid and uses no consumables except a safe, high density, modular electrical power source. A portable and swimmable, self-contained, electrically powered unit (DTPS) has been designed, built, and tested that produces and circulates thermally conditioned water in a closed-loop through a zoned tube suit worn by a diver under a wetsuit to maintain skin and body core temperatures within prescribed safe limits. The system has been validated by using physiological data taken on human subjects over a wide range of ambient water temperatures. Corresponding enthalpy and electrical power measurements were used as the basis of a thermodynamic analysis. The DTPS maintained skin and body core temperatures within safe and functional ranges by providing up to about 200 W of heating in cold water and up to about 330 W of cooling in hot water. The corresponding electrical power consumption was up to about 300 W in cold water and up to about 1500 W in hot water. The results of a complete audit of the power use and heat transfer are presented along with the efficiency of the thermoelectric heating/cooling modules and the duty cycle of the system for a range of water immersion temperatures from to . The DTPS proved to be an effective and reliable apparatus for diver thermal protection in water temperatures from to , which covers most of the range of the earth’s waters. The data presented here can be used to modify the design of the DTPS to meet specific needs of the diving community.