Multivariable plants under input constraints such as actuator saturation are liable to performance deterioration due to control windup and directionality change. A two-stage internal model control (IMC) antiwindup design for open loop stable plants is presented. The design is based on the solution of two low-order quadratic programs at each time step, which addresses both transient and steady-state behaviors of the system. For analyzing the robust stability of such systems against any infinity-norm bounded uncertainty, stability test have also been developed. In particular, we note that the controller input-output mappings satisfy certain integral quadratic constraints. Simulated examples show that the two-stage IMC has superior performance when compared with other existing optimization-based antiwindup methods. The stability test is illustrated for a plant with left matrix fraction uncertainty. A scenario where the proposed two-stage IMC competes favorably with a long prediction horizon model predictive control is described. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3459-3472, 2011