This study was conducted to determine the effects of dietary yeast autolysate on performance, carcass and gut traits, blood parameters, and humoral immune response of broilers. A total of 175 day-old Ross 308 male broiler chicks were allocated into one control group and 4 treatment groups each containing 5 replicate groups of 7 chicks. A basal diet was supplemented with 0, 1, 2, 3, and 4 g/kg of yeast autolysate (Saccharomyces cerevisiae, InteWall) to generate dietary treatments. The experimental period lasted 42 d. Supplemental yeast autolysate improved live weight gain (P < 0.01) and feed conversion (P < 0.001) during the starter period (1 to 21 d). Cumulative FCR was decreased (P < 0.05) during the overall period (1 to 42 d) with 2 and 3 g/kg of yeast autolysate supplementation when compared with the control diet. There were no dietary effects on final live weight, feed intake, excreta pH, excreta moisture, carcass yield, and the relative weight of gizzard, liver, heart, spleen, bursa of Fabricius, and the intestinal weights. Yeast autolysate supplementation decreased relative weight of abdominal fat (P < 0.001) and Escherichia coli count of the digesta (P < 0.01) and increased antibody titers to SRBC (P < 0.001). The pH of jejunal and ileal digesta was decreased at the 2, 3, and 4 g/kg of yeast autolysate supplementation compared with that of birds fed the control diet (P < 0.001). Dietary treatments did not significantly affect blood serum levels of cholesterol, triglyceride, protein, uric acid, aspartate amino transferase, and alanine amino transferase. It was concluded that the dietary supplementation at the level of 2 and 3 g/kg of yeast autolysate was an effective feed additive in broiler feeding because of the increased growth performance, increased immunocompetence, and the reduction of E. coli colonization in the intestine. © Poultry Science Association, Inc.