Humins are a major byproduct formed from acid-catalyzed biomass hydrolysis and are usually considered as a low-value material. In this work, humins were completely degraded by a two-step technology, involving alkaline-catalytic hydrothermal treatment followed by wet oxidation. Using this technology, humins were converted to value-added acetic acid for first time. The highest acetic acid yield of 25.6% on a carbon basis or 37.2% on a mass basis was obtained with a purity of 46.2% based on the organic carbon in the aqueous product. This high-efficiency recovery of acetic acid is a new method for the effective utilization of humins, which is an important finding for biorefineries.