A feeding trial of 8 weeks was performed to evaluate effects of dietary onion peel extract (OPE) supplementation on the growth performance, carcass traits and blood biochemical indicators of male APRI growing rabbits during Egyptian summer environments. Thirty-six weaned male rabbits (646 ± 15.39 g) were randomly allocated to four trial groups (9 rabbits in each group) as a completely randomized design. The first group was fed a control diet without supplementation, while the other three diets supplemented groups with OPE at 200, 350, and 500 mg/kg diet. The findings indicated that dietary supplementation with OPE at levels of 200 or 350 mg/kg diet significantly (P<0.001) enhanced live body weight at 9 and 13 weeks of age compared to the control group. OPE supplementation led to improvements in growth performance indicators including body weight gain, relative growth rate, and feed conversion ratio, whereas feed intake and carcass traits showed no significant changes. The serum cholesterol profile showed favorable changes, with a reduction in total cholesterol, LDL-C, and VLDL-C levels (P≤0.01), and a slight increase in HDL-C (P=0.052), while triglyceride levels remained unchanged. OPE supplementation at levels of 200 or 350 mg/kg resulted in an increase in serum total protein, with the 200 mg/kg diet level significantly enhanced serum albumin (P<0.05) and serum globulin (P<0.01) levels. The antioxidant status was enhanced, evidenced by decreasing malondialdehyde levels (P<0.01) and elevating concentrations of catalase and glutathione, especially at levels of 200 and 500 mg/kg diet. Immunological responses indicated a significant increase in IgM (P<0.05) in the 200 mg/kg group, elevated IgG levels (P<0.05) at 350 mg/kg diet, and enhanced lysozyme activity (P<0.01) at both 200 and 350 mg/kg when compared to control group. Conclusively, from these results it could be concluded that dietary supplements with OPE at levels 200 and 350 mg/kg diet enhanced growth performance, blood cholesterol levels, immune response, and antioxidant activity in fattening ABRI rabbits subjected to heat stress in Egypt.