Prediction or forecasting of the wave excitation force (WEF) is required for implementing a power ef ciency maximisation control of wave energy converters (WECs) due to the inherent non-causality of required power take-off (PTO) force. WEF prediction is a non-trivial challenge, depending on WEF (i) estimation and (ii) forecasting. In this study, an observer-based unknown input estimator (OBUIE) is used to estimate the wave excitation force, then a Gaussian Process (GP) model is adopted to forecast the wave excitation force. A new strategy for combined OBUIE and GP forecasting is presented and the performance of the new scheme is validated on a simulation model of the Wavestar WEC system, considering six different sea states. The simulation results indicate the accuracy and feasibility of the proposed method.