The present study aims to evaluate the efficiency of culturing yeast strains and yeast-like fungi in media derived from the byproducts of the brewing process, such as brewer’s spent grain and its enzymatic hydrolysate. The study used the following feed protein producers: Saccharomyces cerevisiae strains (Y-365, Y-722, and Y-1155) and a strain of yeast-like fungi, Saccharomycopsis fibuligera strain Y-310, from the All-Russian Collection of Industrial Microorganisms. Changes in the number of these microorganisms were studied in the process of their culturing in brewer’s spent grain without the use of a multienzyme composition and in its enzymatic hydrolysate. Bioconversion efficiency for reducing sugars and changes in protein concentration during culturing were determined. It was shown that in order to increase the efficiency of bioconversion of brewer’s spent grain, a pre-treatment of raw materials using a complex of cellulolytic, hemicellulase, and proteolytic enzymes is recommended; the products of enzymolysis are monosaccharides, disaccharides, and amino acids, which are the sources of energy, carbon and nitrogen, necessary for further production of microbial protein. The bioconversion of reducing sugars was experimentally confirmed to have the maximum efficiency of 78.6%; also, a 10.5-fold increase in the amount of protein was observed when culturing Saccharomyces cerevisiae strain Y-365 in the enzyme lysate of brewer’s spent grain. The study results can be used in obtaining feed additives through the biofermentation of brewer’s spent grain with Saccharomyces cerevisiae strain Y-365 and will enable a better use of brewer’s spent grain, increasing its nutritional value and digestibility.