A feeding trial (35 days) was carried out to investigate the effect of Saccharomyces cerevisiae cell wall as a mycotoxin biodegradation agent on the performance, feed efficiency, carcass traits, and immunity response against diseases in broilers fed aflatoxin B1 contaminated diets. For this purpose, 200 one day old broilers were randomly allotted into four groups, each with five replicates (10 birds per replicate). Four starter and finisher experimental rations were formulated by using (A) 0, (B) 1.25, (C) 2.5, and (D) 3.75 g kg−1 of Saccharomyces cerevisiae. Experimental diets were contaminated with aflatoxin B1 (100 ppb kg−1 diet). The experimental chicks were kept under standard managerial conditions, and the vaccination program was followed against infectious bursal disease (IBD), infectious bronchitis (IB), and Newcastle disease (ND) diseases. At the end of the feeding trial, carcass, organ weight, and blood samples were collected randomly to determine the carcass traits and antibody titer against ND and IBD viruses. Throughout the experiment, the addition of 3.75 g kg−1 of the Saccharomyces cerevisiae cell wall (Group-D) in feed resulted in the highest weight gain, final weight, feed intake, and the lowest FCR values followed by C group compared with the other groups. All carcass traits were significantly (p > 0.05) improved by increasing the inclusion levels of Saccharomyces cerevisiae in broiler diets. It could be concluded that the broiler diet supplemented with 2.5 or 3.75 g kg−1 of Saccharomyces cerevisiae as a biodegrading agent resulted in improved growth performance, immunity activity and carcass traits, and supplementation with Saccharomyces cerevisiae at these levels can be used effectively in broiler diets without negatively affecting bird health status