A 20-day trial was conducted to determine the effects of dietary branched-chain amino acids (BCAA) on performance, nutrient digestibility, and gene expression of the mTOR pathway in broiler chicks when exposed to aflatoxin B1 (AFB1). The 6 dietary treatments were arranged in a 2 × 3 factorial with 3 BCAA concentrations (1.16, 1.94, and 2.73%) with or without 1.5 mg/kg AFB1 (1.77 mg/kg analyzed). Each diet was fed to 8 replicate cages (6 chicks per cage) from 6 to 20 d of age. Exposure to AFB1 significantly reduced gain:feed ratio and breast muscle weight (P < 0.05), and tended to decrease cumulative BW gain (P = 0.087), while increasing dietary BCAA improved all performance measures (P ≤ 0.0002), except relative breast muscle weight. Apparent ileal digestibility of N and 9 amino acids were increased by AFB1 (P ≤ 0.05), but were reduced by higher dietary BCAA (P ≤ 0.023). Jejunum histology was not affected by AFB1, while higher dietary BCAA tended to increase villus height (P = 0.08). Additionally, the gene expression of mTOR pathway (mTOR, 4EBP1, and S6K1) from liver and jejunum were not affected by dietary treatments, while muscle expression of S6K1 tended to be increased by AFB1 (P = 0.07). No significant interaction between AFB1 and dietary BCAA were observed for any measures in the current study. Results from this study suggested that feed AFB1 contamination can significantly reduce growth performance and breast muscle growth in broiler chicks at 20 d. Higher BCAA supply may have beneficial impact on bird performance, but this effect is independent of AFB1 exposure.