The U.S. ethanol industry generates more than 40 million metric tons of by-products annually, including whole stillage, thin stillage, wet distillers’ grains, syrup, and distillers dried grains with soluble (DDGS). These materials contain substantial quantities of protein. Especially the hydrophobic prolamin zein yet remain underutilized as low-value feed. This review synthesizes the biochemical composition of major ethanol by-product streams, evaluates classical and emerging extraction technologies, examines industrial integration and sustainability advantages, and analyzes market potential. Protein valorization enables ethanol plants to evolve into diversified biorefineries supplying renewable materials and functional proteins while reducing carbon intensity.