In this paper, we consider the problem of robust controller synthesis with noise spectral uncertainties and nonlinear time‐varying unmodeled dynamics in the LQG (linear‐quadratic‐Gaussian) optimal systems. We derive a robust stabilization criterion for multivariable stochastic systems with nonlinear time‐varying unmodeled dynamics. Meanwhile, we shall frame up an algorithm for synthesizing the robust controller not only to minimize the least favorable cost functionalJbut also to satisfy the robust stabilization criterion via specifying an appropriate weighting scalar in the cost functional. The Wiener‐Hopf's technique in LQG optimal system, the saddle point theory, the Nevanlinna‐Pick interpolation theory, and a property of the contractive operator are employed to treat this problem. Furthermore, there are no stable and minimum phase constraints on the controlled plant.