Brazilian coffee consumption, excluding ready-to-drink beverages, reached 1.1 million tons in 2018, i.e., approximately 16% of the global coffee consumption, surpassing that of the United States and Canada combined. The consumption of roasted and ground coffee still dominates the Brazilian market, representing approximately 80% of all beverage sold in the country. The production of the beverage from this coffee generates coffee grounds (CG), a residue that is invariably discarded. Several studies have assessed the reuse of CG, e.g., as raw material for biofuel production, activated carbon to store anthropogenic gases from industrial processes, coal briquettes, fillers in road landfills etc. Over the next five years, consumption of roasted and ground coffee is expected to continue growing. Therefore, the amount of discarded coffee grounds will also increase. Hence, there is still a need for research into alternative uses of coffee grounds. An alternative for the reuse of this waste is its incorporation in thermoplastic matrices, such as polypropylene. In this investigation we studied the effect of incorporating 10, 20, 30, and 40 wt% CG with a compatibilizer at a 10 wt% content in relation to the residue, using tumble pre-mixing of all components, and subsequent extrusion in a twin-screw extruder. Injection-molded specimens were subjected to mechanical and thermal tests. The incorporation of CG in PP increased the stiffness of all investigated compositions, reaching a 20% increase for the 40wt% composition. However, even with the use of 10 wt% compatibilizer, the CG behaved rather as filler than reinforcement in polypropylene, evidenced by the reduction in tensile strength after incorporation. At low CG levels (10 and 20 wt%) flexural strength slightly increased, about 7%, while at high levels (30 and 40 wt%) impact strength decreased. The largest reduction in impact strength compared to PP was almost 40% for the 30 wt% composition.