In this paper, we propose the concept and design methodology for a resonant reactive shield for the reduction of magnetic field leakage from a wireless power transfer (WPT) systems. By using LC resonance, the reactive shield can generate a cancelling magnetic field to reduce the incident magnetic field from WPT coils and effectively reduce the total magnetic field without consuming additional power. The shielding effectiveness of the resonant reactive shield and its effect on WPT efficiency are analyzed with simulation and measurements. For practical application to wirelessly charged electric vehicles, an automatic tuning system for the resonant reactive shield is also proposed and implemented. The effectiveness of a resonant reactive shielding is verified by experiments in a wirelessly charged electric bus.