Recycled polyethylene terephthalate (rPET) is widely considered a sustainable alternative to virgin polyethylene terephthalate (PET). Nevertheless, its application in value-added processes is frequently restricted due to property degradation occurring during recycling. This study investigates the use of slate dust (SD) as a reinforcing filler and its effects on the structural, thermal, and thermomechanical properties of rPET. Twin-screw extrusion and injection molding were used to prepare composites with up to 20 wt% of SD. Gel permeation chromatography, Fourier transform infrared spectroscopy, and x-ray diffraction confirmed that SD maintains chemical inertness during processing, with no detectable molecular degradation due to the filler. With a 20 wt% SD content, the thermal conductivity of rPET increased by approximately 41%, from 0.267 to 0.377 W/mK. Additionally, the crystallization temperature increased by 5°C, indicating the filler's nucleating effect. Dynamic mechanical analysis (DMA) indicates a ~39% increase in storage modulus at room temperature. To further explore the filler-matrix interaction, Cole-Cole plots and C-factor, reinforcement efficiency, degree of entanglement, and adhesion-related factors (A and B factors) were calculated from the DMA results. This study promotes waste valorization strategies by using slate waste as a filler and enhancing rPET to develop recycled polymer composites aligned with circular economy principles.