Waste foundry sand (WFS), the disregarded molding sand from the casting industry, is a byproduct with increasing economic and environmental impact due to landfill maintenance costs and stricter regulations. Only a small fraction (11%) of WFS is currently reused. In this work a novel valorization pathway for WFS is proposed, as a thermal energy storage material. WFS is mixed with molten NaNO₃ and clay and then sintered, to produce a composite phase change material (CPCM). The fabricated CPCM is found to have good thermal and structural stability up to 400 °C. Its optimal composition with a mass ratio of 0.6-0.3-0.1 NaNO₃-WFS-Clay has an specific energy density of 628±27 kJ/kg and an average thermal conductivity is 1.38 W/mK in a temperature range of 25-400 °C. The CPCM has good mechanical strength and a low coefficient of thermal expansion, compared to that of NaNO₃. This material can be applied in medium-high temperature thermal energy storage and waste heat recovery applications, increasing the upcycling potential of WFS in a sustainable manner.