This paper investigates the decentralized asymptotic stabilization problem for a class of nonlinear interconnected systems with unmatched uncertainties by the Lyapunov stability theory and the variable structure system theory. A robust stability condition of the sliding mode and a new robust decentralized sliding controller for each subsystem are derived, such that the local asymptotic stability of the sliding mode and the global asymptotic stability of the composite sliding surface are guaranteed. This stability condition need not solve any Lyapunov equation, which only relates to local subsystems with reduced order. The nonlinearities, parameter variations and interconnections need not satisfy the so-called ''matching condition.'' We have shown that any interconnected system with unmatched uncertainties using the proposed decentralized sliding mode controller will be globally asymptotically stable, as long as unmatched uncertainties are within some bound. Finally, a two-pendulum system is given to illustrate our results.