A 17-run Box–Behnken design with three factors and three levels, including five replicates at the center point, was used to fit a second-order response surface and optimize the ultrasound-L-cysteine (US-L-cys) process conditions for inhibiting the polyphenol oxidase (PPO) activity of fresh-cut potatoes. The R 2 value was 0.9725, indicating that the model fitted the regression model well. The optimal US-L-cys conditions were as follows: 2 g L −1 L-cys concentration, 6 min US time, and 360 W US power. The mean PPO activity under optimal US-L-cys conditions was 7.506 U g −1 , which was in accordance with the values predicted by the model. The fresh-cut samples treated with US-L-cys still maintained the lowest browning index and PPO activity during 48 h of storage in comparison with the control and unitary-treated fresh-cut samples. The findings suggest that US-L-cys treatment was a useful means to inhibit enzymatic browning in fresh-cut potatoes.