In this paper, a method for determining the modal characteristics of rectangular composite sandwich plates with different internal masses and flexible supports is presented. The layer-wise linear displacement theory is used to simulate the displacements in the composite sandwich plate. The elastic edge and/or internal supports are modeled as equivalent distributed spring systems. The Rayleigh-Ritz method is then used to determine the modal characteristics of the flexibly supported composite sandwich plate. The free vibration analyses of a number of flexibly supported composite sandwich plates with different types of distributed masses are performed using the proposed method. The effects of the stiffness of the flexible supports and the locations of the masses on the modal characteristics of the composite sandwich plates are studied. It has been shown that the elastic restraints have significant effects on the lower natural frequencies of the composite plate and such effects diminish as the mode number of the plate increases.