The formation of a tribo-film on the counterface plays an important role on the tribological performances of polymer subjected to mixed and boundary lubrication conditions. However, when freshwater is used as a lubricant, the formation of a tribo-film usually is hindered. In order to overcome this disadvantage, PEEK/α-FeOOH nanocomposites were developed and their tribological performances were studied under water lubrication conditions in the present work. It was demonstrated that the inclusion of α-FeOOH nanoparticles (NPs) into the PEEK matrix improves significantly the tribological performance of the matrix. The nanostructures and properties of the tribo-films formed on the steel counterface were comprehensively studied. It was identified that the addition of α-FeOOH NPs promotes the formation of a lubricating tribo-film which covers the entire counterface. Based on the investigations on tribo-films, we deem that the α-FeOOH NPs act as precursors for the dehydration reaction promoting the formation of a tribo-film which consists of α-Fe₂O₃ and transferred PEEK. It is assumed that the enhanced tribological performance is related to the possibly high load-bearing capability and “easy-to-shear” characteristic of the tribo-film.