Water transport in a polymer electrolyte membrane used for a fuel cell was investigated using magnetic resonance imaging. Flowing humidified gases enabled visualization of two-dimensional water content distributions within the membrane under water permeation processes with different water activity across the membrane. Magnetic resonance imaging visualization revealed that both homogeneous and inhomogeneous distributions of water contents were observed within the membrane. It was also found that the rate-determining process of water permeation across the membrane varied according to the water activity difference corresponding to the diffusion process through the membrane and the interfacial transport process across the membrane-gas interface.