Impedance measurements on the interlayer region in an organic coating system subjected to cyclic mechanical stress are presented. A novel impedance sensor capable of detecting contact imperfections between adjacent coating layers has been developed. A bilayered coating system designated to serve in a marine environment has been subjected to conditions simulating synergistic impact of fatigue effects due to repetitive mechanical stress and immersion in 3% NaCl aqueous solution. The results revealed the detrimental influence of cyclic stress and water ingress on the quality of the interlayer adhesion. The proposed method of interlayer impedance evaluation can be employed to interlayer defect monitoring in multi-coating systems.