This study proposed a painted flexible piezoelectric device (FPED) for harvesting wave energy. The painted FPED had a laminated structure consisting of an elastic material and a piezoelectric paint. The device could be easily deformed by an external force such as wind, wave, current, or mechanical vibration, and it could generate electrical power from an ambient, non-utilised energy source. The results revealed the mechanical deformation and electrical performance of the painted FPED excited by wave conditions. They also clarified the influence of some key parameters, such as the submerged depth, wave steepness, aspect ratio, actuator pattern (unimorph and bimorph), and thickness of the painted piezo-material and elastomer on the output voltage generated by a painted FPED excited by various wave forces, heights and periods. Moreover, a theoretical model was proposed, and the results were in good agreement with the experimental results under several wave conditions. The electrical power in real sea states was estimated by the theoretical model. The electric power could be stored in batteries, and both the monitoring sensors and ocean environmental devices could be triggered and activated to work for practical applications in real sea states.