Rapeseed meal (RM) is an important agroindustrial by-product produced in large quantities by oil extraction from seeds. However, the efficient utilization of RM as animal feed is limited by its low metabolizable energy, poor palatability, and high levels of fiber and anti-nutritional components. Here, we investigate the potential of enriching RM with single-cell protein by fermentation with conventional and unconventional yeasts, to make a nutritionally improved feed component. The process of simultaneous saccharification and fermentation improved the parameters of the waste biomass, especially the protein content, the amount of crude fiber, and the degree of biotransformation of isoflavone compounds present in the waste material. Fermentation yielded the highest protein gain for the Saccharomyces cerevisiae Ethanol Red strain (ΔN=2.38%) at a biomass load of 12.5 g and for Scheffersomyces stipitis (ΔN=2.34%) at an enzyme dose of 0.125ml/10g DM. Crude fiber content (CF) was reduced by 2.55–7.18%. The SSF process resulted in the conversion of isoflavones to forms with fewer adverse effects and lower estrogenic activity.