The present study aimed to investigate the effects of the dietary modified dried vinasse (MDV, LayMass®), a byproduct of the bakers’ yeast industry, as an alternative protein source in aged laying hens. A total of 64 Brown Nick laying hens aged 65 weeks were randomly allocated to four experimental groups and fed diets containing 0, 3, 6, and 9% MDV for 12 weeks. Dietary inclusion of MDV had no significant effects on egg production, feed efficiency, egg weight, egg quality traits, egg mineral content, egg lipid composition, total unsaturated and polyunsaturated fatty acids in egg yolk, and tibia characteristics. However, serum total antioxidant status, catalase activity, lysozyme activity, and total antibody titres against sheep red blood cells (SRBCs) increased linearly with increasing MDV levels in the diets. In conclusion, it could be indicated that MDV may serve as a sustainable alternative protein source, offering immunological and antioxidative benefits without adverse effects on performance and egg quality in aged laying hens. Therefore, MDV may partially replace soybean meal at levels up to 9% in layer diets, contributing to sustainability in the egg industry.