This paper describes the concept, detailed design, fabrication, and test results of a Reversible Thermal Panel (RTP), which is a new, passive and lightweight 100W-class deployable radiator with an environment adaptive function. The RTP changes its function reversibly from a radiator to a solar absorber by deploying/stowing the radiator/absorber reversible fin upon changes in the heat dissipation and thermal environment, and is effective for the thermal control of high power density small satellites, landers, and interplanetary spacecrafts. Parametric studies were conducted with detailed simulation models, and the RTP configuration that satisfies thermal requirements with the restraints of weight and fin efficiency variation was determined. Based on the analytical results, an RTP engineering model was fabricated using high thermal conductive graphite sheets in the reversible fin, and a shape memory alloy in the passive deployment/stowing actuator. Test results indicated excellent performance of the RTP as a passive radiator from the standpoints of heat rejection capability, specific heat rejection, and variation of the fin efficiency. The effectiveness of the RTP as a solar absorber that can be substituted for a survival heater was also demonstrated.