Spent coffee grounds (SCG) represent a high volume agro-industrial waste stream generated globally, and their valorization as biofillers in thermoplastic composites offers a promising pathway for waste reduction and sustainable material development. This study examines the influence of alkali treatment and filler loading (5, 10, 15, and 20 wt%) on the physical, rheological, and mechanical behavior of polypropylene (PP) composites reinforced with SCG. Composites were produced by injection molding and characterized in terms of density, melt flow index (MFI), Shore D hardness, and tensile properties. One-way ANOVA (Analysis of variance) confirmed statistically significant differences among all formulations for every measured property (p < 0.05), and Tukey's Honestly Significant Difference (HSD) post hoc tests (α = 0.05) were applied to identify homogeneous subsets. Alkali treatment improved SCG surface roughness and interfacial adhesion with the PP matrix, as confirmed by optical microscopy. Mechanically, treated composites retained tensile strength close to neat PP at low filler loadings (38.6-40.5 MPa at 5–10 wt%), while hardness increased progressively, reaching 66.96 Shore D at 20 wt%. The MFI dropped by approximately 31% for untreated composites and approximately 75% for treated ones at 20 wt%, relative to neat PP (17.03 g/10 min). Composite density increased by up to 6.6% with SCG addition. Four regression models were fitted to each measured property; quadratic models yielded the highest R² values for most parameters, while elongation at break followed a linear trend. Treated composites consistently showed higher R² values than untreated ones, confirming that surface treatment improves the predictability of composite behavior. These results demonstrate that SCG can be effectively valorized as an eco-friendly biofiller in PP-based materials, contributing to both waste reduction and the development of more sustainable composites.