Delayed Detached Eddy Simulation of supersonic flutter of a 3D wing is conducted at free stream Mach number of 1.141 using a fully coupled fluid/structure interaction (FSI). Unsteady 3D compressible Navier-Stokes equations are solved with a system of 5 decoupled structure modal equations in a fully coupled manner. The low diffusion E-CUSP scheme with a 5th order WENO reconstruction for the inviscid flux and a set of 4th order central differencing for the viscous terms are used to accurately capture the shock wave/turbulent boundary layer interaction of the vibrating wing. The predicted flutter boundary at supersonic Mach number achieves excellent agreement with experiment. It appears to be the first time that a numerical prediction of supersonic flutter boundary matches with experiment accurately.