This work deals with the nutritional evaluation of grape marc hydrolyzates as fermentation medium for Lactobacillus pentosus. Usually, the fermentation of xylose and arabinose in the presence of glucose remains a primary obstacle for economical biomass conversion. The few microorganisms that can grow simultaneously on both pentose and hexose sugars contained in lignocellulosic feedstocks typically grow slowly and demonstrate marginal yields and productivities. Moreover, lignocellulosic hydrolyzates contain phenolic compounds and other components originated by the degradation of sugars that can inhibit lactic acid fermentation. However, in this case, grape marc hydrolyzates not only did not need a detoxification stage, but it also improved the xylose consumption by Lactobacillus pentosus with a faster and more efficient conversion of hemicellulosic sugars compared with synthetic media. After analysis of grape marc hydrolyzates, it was observed that minerals such as K (2,707 mg/L), Ca (3,681 mg/L), and Mg (198.5 mg/L) are present in higher concentration than those found in the general medium of Lactobacillus (1,705 mg/L of K, 58.3 mg/L of Ca, and 27.0 mg/L of Mg). Moreover, grape marc hydrolyzates contain an additional source of nitrogen (9.2 g/L) which, together with their elevated mineral concentration, improved lactic acid fermentation compared with synthetic media.