This paper is concerned with an active wave control method of a rectangular panel. It is the purpose of this paper to present a wave filtering method for the panel using smart modal sensors and its application to an adaptive feedforward control system. Firstly, a wave solution of a rectangular panel is derived to describe the wave dynamics of the structure. This is followed by the proposition of the design procedure of the wave filter using smart mode sensors. Then, from a viewpoint of numerical analyses, accuracy of the proposed method is verified using condition numbers of a filtering matrix. Furthermore, a multi-rate technique, which reduces the computational burden, is introduced for approximation of the sub-filters in the wave filter. Finally, experiment on an adaptive feedforward control system using the proposed method is carried out. It is found that the reflected wave absorbing control enables the inactivation of vibration modes.