This paper presents both state space and frequency domain approaches for the decentralized decoupling of multivariable control systems with unmodelled interaction. The controller has a configuration of two degrees of freedom. Thenominal controlleris tuned to satisfy the eigenvalue specification of the isolated single variable subsystem. The rejection of unmodelled coupling due to the connections or disconnections of other subsystems is controlled by thedecentralized compensator.The method assumes that only local information is acquirable, no information from other subsystems and no interconnection parameters are required. This gives the convenience of single variable design for multi‐variable control applications. The decentralized compensator can also be used to improve the performance of the control system with unmodelled nonlinearity or parameter drifting. The stability of the control system with the decentralized compensator has been investigated. Two numerical examples are given to show the feasibility of the method.