This study evaluated the influence of dietary supplementation of onionskin waste (OW) and synthetic additives on growth, oxidative status, blood chemistry, caecal bacteria, carcase, immune indices and meat quality in broilers. One day old Ross 308 chicks (n ¼ 280) were randomly allotted to basal diets containing either no additive, B-1; 0.5 g/kg oxytetracyline þ 0.13 g/kg tertbutylhydroxyanisole, B-2; 10 g/kg OW, B-3; or 20 g/kg OW, B-4 for 42 days. Supplemented birds had higher (p < .05) body weight gain (BWG) and carcase weight compared to the B-1 birds. Diets had no effect (p > .05) on feed intake and feed efficiency, hematological indices, organ weight and serum immunoglobulin M. Onionskin supplementation repressed (p < .05) abdominal fat, serum total cholesterol, triglycerides, aspartate aminotransferase, muscle cholesterol and splenic interleukin-1b. The additives up-regulated (p < .05) splenic intelukin-10 and down-regulated tumour necrosis factor-a, and serum immunoglobulin A. Caecal Lactobacillus spp. was higher (p < .05) in B-1 and B-3 birds compared with the B-2 and B-4 birds. Supplemented birds had lower (p < .05) caecal Escherichia coli and Salmonella spp. counts than the B-1 birds. Antioxidant enzymes and total antioxidant capacity of breast muscle, liver and serum were higher (p < .05) in the supplemented birds compared with the B-1 birds. On day 1 post-mortem, breast meat malondialdehyde and carbonyl contents were lower (p < .05) in the supplemented birds compared with the B-1 birds. Dietary supplementation with 20 g/kg OW improved BWG and tissue antioxidant status, beneficially altered immune status, and caecal bacteria counts, and reduced abdominal fat, tissue cholesterol, and meat oxidative deterioration in broiler chickens. HIGHLIGHTSOnionskin supplementation improved body and carcase weights and lowered serum and meat cholesterol in broilers. Onionskin enhanced tissue antioxidant enzymes and reduced oxidative deterioration in broilers. Onionskin supplementation altered caecal bacteria and immune status in broilers.