Similar to an electrical diode, a thermal diode/switch is a device which allows heat to flow to a preferential direction. While a number of different types of thermal diodes/switches exist, a phase-change thermal diode/switch yields a greater diodicity performance compared to other types. However, most phase-change thermal diodes/switches have complex designs, complicated fabrication processes, and sophisticated working principles. Moreover, some materials used in thermal diodes/switches are rare, expensive and toxic. Thus, in this study, a simple water-vapor chamber thermal diode using latent heat of vaporization is designed, assembled and investigated, both experimentally and theoretically. The effects of the temperature difference between the hot side and the cold side and the water-air volume ratio on the effective thermal conductivity and diodicity of the water-vapor chamber thermal diode are also investigated. Mathematical models are also developed, not only for predicting one-dimensional phase change heat transfer performance in a rigid enclosure, but also for verifying the results from the experiment. This is the first study in which the water-vapor chamber thermal diode is studied both experimentally and theoretically. The experimental results show that an increase in temperature at the hot side of the thermal diode significantly enhances the performance of the thermal diode. The effective thermal conductivity at the hot side temperature of 70 °C shows a 50% improvement when compared with that at the hot side temperature of 40 °C in the forward direction. Additionally, with the water-air volume ratio of 0.5, the maximum diodicity of 1.43 is reported. Moreover, it is also found that the water-vapor volume ratio affects heat transfer performance and diodicity of the water-vapor chamber thermal diode. The results of these findings can further advance knowledge on phase-change heat transfer and can be applied to several applications, including heat transfer enhancement, waste heat power systems, thermal management and solar thermoelectric power generation.