This study investigates the thermal, thermomechanical, and structural properties of recycled polyethylene terephthalate (rPET)/waste marble dust (MD) composites. The research aims to evaluate the potential of using waste marble dust as a reinforcing agent in rPET-based composites to develop environmentally friendly materials. rPET composites with MD content ranging from 0 to 20 wt% were fabricated through extrusion and injection molding. The composites were characterized using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). The DSC results showed that MD acted as a nucleating agent, increasing the crystallization temperature of rPET from 196.7°C to 204.4°C, while also reducing the crystallinity ratio from 23.7% to 19.2%. TGA measurements indicated a slight increase in the thermal stability of rPET, with its decomposition temperature increasing from 446°C to 451°C when 20 wt% MD was incorporated. DMA results showed that MD-filled composites exhibited improved stiffness across the entire temperature range compared to neat rPET. Several factors derived from DMA data, including the effectiveness factor, degree of entanglement, and reinforcing efficiency factor, suggested a good interaction between the components, indicating a proper reinforcing ability of marble powder. However, above 5 wt% MD content, the reinforcing efficiency deteriorated due to filler agglomeration, as supported by scanning electron microscopic images. The FTIR analysis showed no alteration in the molecular structure of the components. The thermal properties of the composites were influenced by the MD content, with the thermal stability of rPET slightly increasing. The dynamic mechanical properties of the composites were also affected by the MD content, with the storage modulus increasing as the MD content increased. The results indicated that the interaction between the MD particles and the rPET matrix was significant, with the best reinforcement effect observed at 5 wt% MD content. However, above this level, the mechanical properties of the composites deteriorated due to agglomeration of the filler particles. The study concludes that the use of waste marble dust as a filler in rPET composites can improve the mechanical and thermal properties of the composites, but excessive filler content can lead to agglomeration and reduced performance.This study investigates the thermal, thermomechanical, and structural properties of recycled polyethylene terephthalate (rPET)/waste marble dust (MD) composites. The research aims to evaluate the potential of using waste marble dust as a reinforcing agent in rPET-based composites to develop environmentally friendly materials. rPET composites with MD content ranging from 0 to 20 wt% were fabricated through extrusion and injection molding. The composites were characterized using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). The DSC results showed that MD acted as a nucleating agent, increasing the crystallization temperature of rPET from 196.7°C to 204.4°C, while also reducing the crystallinity ratio from 23.7% to 19.2%. TGA measurements indicated a slight increase in the thermal stability of rPET, with its decomposition temperature increasing from 446°C to 451°C when 20 wt% MD was incorporated. DMA results showed that MD-filled composites exhibited improved stiffness across the entire temperature range compared to neat rPET. Several factors derived from DMA data, including the effectiveness factor, degree of entanglement, and reinforcing efficiency factor, suggested a good interaction between the components, indicating a proper reinforcing ability of marble powder. However, above 5 wt% MD content, the reinforcing efficiency deteriorated due to filler agglomeration, as supported by scanning electron microscopic images. The FTIR analysis showed no alteration in the molecular structure of the components. The thermal properties of the composites were influenced by the MD content, with the thermal stability of rPET slightly increasing. The dynamic mechanical properties of the composites were also affected by the MD content, with the storage modulus increasing as the MD content increased. The results indicated that the interaction between the MD particles and the rPET matrix was significant, with the best reinforcement effect observed at 5 wt% MD content. However, above this level, the mechanical properties of the composites deteriorated due to agglomeration of the filler particles. The study concludes that the use of waste marble dust as a filler in rPET composites can improve the mechanical and thermal properties of the composites, but excessive filler content can lead to agglomeration and reduced performance.