In the field of solar thermal storage systems, metallurgical industry by-products have been proposed as filler materials for packed-bed thermal energy storage due to their low cost and suitable thermophysical properties. One of these by-products is copper slag, which has emerged as a competitive option compared to other types of industrial by-products. However, further research of its properties, composition, and heterogeneity is needed to fully address its potential as a storage medium. Approximately 2.2 tons of copper slag is produced per ton of copper extracted, posing disposal challenges for mining companies. Hence, there is growing interest in finding secondary uses for these slags. The present study investigated samples of copper slag from a Chilean foundry disposal site. Elemental and mineral characterization revealed that this heterogeneous material has high iron content with both amorphous and crystalline phases present. The evaluation of thermophysical properties showed stable specific heat capacity that increases with temperature within the range of 100 °C to 450 °C. However, these profiles exhibit variability in heat capacity, particularly at higher temperatures, which decreases with subsequent heating cycles. The results suggest that copper slag has potential as an alternative material for sensible heat storage in packed-bed systems, nonetheless, assessing the variability of its thermophysical properties is crucial to establish its feasibility for sustainable energy solutions.