Autohydrolysis, which is an eco-friendly technology that employs only water as extraction solvent, was used to extract antioxidant phenolic compounds from spent coffee grounds (SCG). Experimental assays were carried out using different temperatures (160–200 °C), liquid/solid ratios (5–15 ml/g SCG) and extraction times (10–50 min) in order to determine the conditions that maximize the extraction results. The optimum conditions to produce extracts with high content of phenolic compounds (40.36 mg GAE/g SCG) and high antioxidant activity (FRAP = 69.50 mg Fe(II)/g SCG, DPPH = 28.15 mg TE/g SCG, ABTS = 31.46 mg TE/g SCG, and TAA = 66.21 mg α-TOC/g SCG) consisted in using 15 ml water/g SCG, at 200 °C during 50 min. Apart from being a green technology, autohydrolysis under optimized conditions was demonstrated to be an efficient method to extract antioxidant phenolic compounds from SCG.