Aiming at the average receiver complexity minimization, in this paper we advocate an adaptive detector for the spatial modulation (SM) multiple-inputmultiple-output (MIMO) system. In the proposed adaptive detector, several SM-MIMO detection algorithms are embedded and switched on-The-fly depending on the channel conditions. By intelligently switching the one with minimum complexity and satisfying the just-Acceptable error rate (JAER) constraint, the average detection complexity is greatly saved. We commence with two different K-best sphere decoders, which have different preferred operating regions. The detectorswitching strategy is then designed to efficiently switch and adjust these two algorithms. The simulation results demonstrate that, the proposed adaptive MIMO detector enjoys the widest operating region as the optimal detector with much less average computational complexity.