This paper deals with the problem of robust L₂ disturbance attenuation for nonlinear systems with input dynamical uncertainty. The input dynamical uncertainty is restricted to be minimum-phase and relative degree zero. A sufficient condition is given such that the nonlinear system satisfies the L₂ gain performance and input-to-state stable (ISS) property. Using this condition, a design approach is given for smooth state feedback control law that solves the robust L₂ disturbance attenuation problem, and the approach is extended to more general case where the nominal system has higher relative degree. Finally, a numerical example is given to demonstrate the proposed approach.