In this paper, a computational methodology for buffet loads prediction has been developed and validated with wind tunnel and flight test data. A key component of this computational methodology is the generalized aerodynamic forces due to buffet computed by the FUN3D code with Detached Eddy Simulation (DES) developed by NASA Langley. By comparing the FUN3D DES results with other computational solutions and wind tunnel measurements on rigid F-15 models, it is verified that FUN3D DES can provide the accurate random pressure fields occurring in massively separated flow creating buffet. The flight test data of an F-15 at 6 high angle-of-attack flight conditions are selected to validate the computational methodology for buffet loads prediction. Several key findings of this investigation are that the majority of the damping in the system originates from the aerodynamic feedback forces to reduce the magnitude of the response; the structural damping has a smaller effect on the structural response amplitudes. The power spectral densities of the structural response predicted by this computational methodology at the 6 flight conditions correlate reasonably well with the flight test data; showing that this computational methodology is a viable tool for the prediction of the structural response to buffet loads.