While other extraction methods have been tempted, a microwave-assisted extraction (MAE) method coupled with the orthogonal array design was investigated for efficient extraction of the phenolic compounds in potato downstream wastes. Four parameters were examined for the MAE of the total phenolic content (TPC) and optimized at 60% ethanol, 80°C, 2 min, solid-to-solvent ratio 1:40 (g/ml). The MAE was proven more efficient than the conventional solvent extraction by refluxing. The optimized model showed that the downstream wastes, both the supernatant and the residue contained high TPC, particularly the former (11.0 ± 0.26 mg GAE/g DW). The antioxidant activities (DPPH and FRAP) closely correlated with the TPC of the samples (r = 0.92-0.97). Chlorogenic acid and caffeic acid were found to be the predominant phenolic acids. The extracts of the downstream wastes from potato processing can be a promising candidate for functional foods and nutraceutical ingredients.