Abstract: Hydrogen has the advantages of high energy density, no pollution, and long-term storage. As an important medium for the transformation of energy interconnection, it helps to promote the clean and efficient use of traditional fossil energy, support the large-scale development of renewable energy, and achieve large-scale deep decarbonization. With the excellent responsiveness to intermittent and fluctuating power supplies, proton exchange membrane (PEM) water electrolysis has been a research hotspot in the field of hydrogen production with renewable energy and will be one of the main technical routes for effective hydrogen supply in the future. The high-pressure operation of PEM electrolyzers has been successfully realized and commercialized, considering PEM’s outstanding mechanical strength and gas separation properties. However, due to the water-absorbing properties of the membrane, an important problem in high-pressure PEM water electrolysis (especially under differential pressure conditions and high pressure in the cathode compartment/atmospheric pressure in the anode compartment) is the permeation of hydrogen through the membrane, which affects safety and efficiency. In this article, the research progress of hydrogen permeation in PEM electrolysis was reviewed. First, the theory of permeation was introduced. Second, considering the relationship among the permeation flux, permeability, and partial pressure difference described by Fick’s law, the effects of temperature/pressure, hydration degree of the membrane, and partial pressure difference on hydrogen permeation were reviewed. In the normal operating pressure range (<3.5 MPa) for hydrogen production through PEM electrolysis, the diffusion coefficient and solubility are mainly affected by temperature, and the permeability increases with increasing temperature. The permeability of hydrogen in water is approximately 5–10 times that of a dry film, and the permeability increases with increases in the relative humidity of the membrane. The influence of partial pressure difference in hydrogen permeation shows a linear dependence in permeation cell and quadratic dependence in real electrolysis. The quadratic dependence may be attributed to the convective permeation caused by the increase in membrane water permeability and changes in the water channel. Third, considering the current in real operating conditions of electrolysis, the effect of current density on hydrogen permeation was reviewed. The permeability increases with the increase in current density, which may be attributed to the increase in hydrogen supersaturation in the cathode. At a high current density, the hydrogen concentration within the ionomer at the cathode catalyst particles become higher, and the high concentration gradient causes hydrogen to diffuse from cathode to anode.