BACKGROUND The disadvantages associated with the use of synthetic additives in animal production could threaten human and animal health, and the safety of animal‐derived foods. This study assessed the growth performance, blood chemistry, immune indices, selected caeca bacterial population, muscle antioxidant enzyme activities, and meat quality in broiler chickens fed diet supplemented with antibiotic (70% oxytetracycline +30% neomycin), tert‐butylhydroxytoluene or onion leaf powder (OLP). One day old Ross 308 chicks (n = 240) were assigned randomly to either D‐1, control diet (CD) without additives; D‐2, CD + 0.3 g kg−1 antibiotic +0.15 g kg−1 tert‐butylhydroxytoluene; D‐3, CD + 2.5 g kg−1 OLP; or D‐4, CD + 5 g kg−1 OLP for 42 days. RESULTS The D‐2 and D‐4 diets improved (P < 0.05) bodyweight gain and feed efficiency in broilers. Platelet and cecal Lactobacillus spp. counts were higher (P < 0.05) whereas muscle cholesterol was lower (P < 0.05) in the OLP‐supplemented birds. Supplemented birds had higher (P < 0.01) splenic interleukin‐10 and lower (P < 0.01) splenic tumor necrosis factor‐α, immunoglobulin A, cecal E. coli and C. perfringens counts compared with the D‐1 birds. The D‐4 birds had the least (P < 0.05) splenic interleukin‐1β. Dietary supplements increased (P < 0.05) catalase, glutathione peroxidase, and total antioxidant capacity, and lowered (P < 0.05) drip loss, malondialdehyde and carbonyl content in breast meat. CONCLUSION Dietary supplementation of 5 g kg−1 OLP exerted antimicrobial, immunomodulatory, and antioxidant effects that were comparable to those of antibiotics and tert‐butylhydroxytoluene in broiler chickens. © 2021 Society of Chemical Industry.