Polyetheretherketone (PEEK) is commonly used in power connectors due to its excellent mechanical and electrical properties. In submarine electrical components, water molecules can diffuse through polymeric sealings and interfaces and finally increase the humidity in the main electrical insulation system. To examine the effect of water sorption on the dielectric properties of a semi-crystalline commercially available PEEK, polarization and depolarization currents were measured on samples preconditioned in a climate chamber to either a dry state or to a water concentration of 5000 ppmw at 30, 60 and 90 °C. The water absorption process was characterized and found to follow Fick’s law, with the solubility decreasing with increasing temperature. The absorption of water was found to strongly increase the electrical conductivity. At 90 °C the conductivity increased by 85 times compared to the value measured on dry conditioned samples. The conductivity of dry PEEK also increases less with temperature than that of wet PEEK, indicating a lower activation energy in the absence of water molecules. The conduction mechanism in wet PEEK insulation is likely ionic hopping.