Aiming for sustainable construction and green building with less greenhouse gas emissions and less energy consumption, the authors proposed a new composite material based on gray clay sourced from the Gharb region of Morocco and Cactus seeds, a by-product of cosmetic and pharmaceutical industries, utilized for the first time in the construction sector as an ecological additive. This study focuses on the influence of incorporating this gray clay with cactus seeds, on the material’s mechanical and thermal properties. The thermal properties, thermal conductivity, thermal effusivity, and thermal capacity of this material are investigated using the hot plane method in steady state and in transient regime. As for mechanical properties, the compression and flexural strengths are determined using a uniaxial compression test and a three-point bending test. The results of adding 50% of cactus seeds to this clay show a great thermal potential reducing thermal properties nearly by half. However, the mechanical strengths have decreased in comparison with the reference material (clay alone).