The demand for cheap, healthy, and sustainable alternative protein sources has turned research interest into microbial proteins. Mycoproteins prevail due to their quite balanced amino acid profile, low carbon footprint and high sustainability potential. The goal of this research was to investigate the capability ofPleurotus ostreatusto metabolize the main sugars of agro-industrial side streams, such as aspen wood chips hydrolysate, to produce high-value protein with low cost. Our results indicate thatP. ostreatusLGAM 1123 could be cultivated both in a C-6 (glucose)- and C-5(xylose)-sugar-containing medium for mycoprotein production. A mixture of glucose and xylose was found to be ideal for biomass production with high protein content and rich amino acid profile.P. ostreatusLGAM 1123 cultivation in a 4 L stirred-tank bioreactor using aspen hydrolysate was achieved with 25.0 ± 3.4 g L−1biomass production, 1.8 ± 0.4 d−1specific growth rate and a protein yield of 54.5 ± 0.5% (g/100 g sugars). PCA analysis of the amino acids revealed a strong correlation between the amino acid composition of the protein produced and the ratios of glucose and xylose in the culture medium. The production of high-nutrient mycoprotein by submerged fermentation of the edible fungusP. ostreatususing agro-industrial hydrolysates is a promising bioprocess in the food and feed industry.