Spent coffee grounds are one of the most interesting waste streams of coffee industry containing antioxidants, whose extraction may represent an innovative solution for the reduction of the environmental burden of this waste, simultaneously providing high added value compounds. The aim of this work was to valorize SCG (Coffea arabica L.) using a non-conventional technique (high pressure and temperature extraction), by providing pivotal information and experimental data essential for the design of a profitable and green process. A 3-level factorial design was firstly employed to verify the combined effects of ethanol concentration in water, used as solvent, and temperature on total polyphenol yield and antiradical power of the extract. Desirability method was used to estimate the optimal conditions (ethanol 50% (v/v) as solvent at 150 °C), allowing the recovery of remarkable amount of bioactive compounds (30.4 ± 1.7 mg/100 mL of chlorogenic acid and 31.7 ± 2.9 mg/100 mL of caffeine). Moreover, Peleg's model was successfully applied to describe extraction kinetic, showing that an extraction time of 60 min is sufficient to produce extracts with high extraction yields (43 ± 0.8 mg caffeic acid equivalents/g dried biomass) and remarkable antiradical properties (734 ± 11 μg trolox equivalents/g dried biomass).