Recently, there has been an ever-growing demand for multi-receiver wireless power transfer (WPT) systems with a high power transfer efficiency. In this paper, a novel optimization-based method is proposed to precisely determine the optimal resonant condition for multi-receiver WPT systems in order to maximize the power transfer efficiency while satisfying all the rated power that can be differently required in each receiver module. The proposed method links the optimization module and the analysis module to iteratively update design variables during optimization based on electric performance evaluated in the analysis module. After being verified with a “single-transmitter-to-a-single-receiver” WPT system which has a well-known resonance condition, the proposed method is applied to determine the optimal resonant condition for various “single-transmitter-to-multiple-receivers” WPT systems in both initial and disturbed configurations. The experimental validation demonstrates the performance and potential of the proposed method to compensate for coupling changes of multi-receiver WPT systems.