This paper proposes a new gradient-based coil layout optimization method that can determine the optimal receiver coils to maximize the power transfer efficiency of a multitransmitter wireless power transfer (WPT) system. The smooth boundary-based coil representation allows for a precise change of coil layout during optimization, thereby more accurately evaluating the performance of the WPT system. The power transfer capacity, power transfer efficiency, and coil mass are mathematically expressed in terms of design variables (i.e., coil turns for the transmitter and receiver, and input voltage), and their analytical sensitivities are derived with respect to design variables to efficiently conduct gradient-based optimization. Through the proposed coil layout optimization, the optimal receiver coils for the wireless portable charger are determined to maximize the power transfer efficiency while satisfying the given power transfer capacity and coil mass requirements. Then, the experiments are conducted under the same working conditions to prove the validity of the proposed method.