© 2016 IEEE.In this letter, we propose an efficient structured approximate message passing (Str-AMP) detector for multi-user spatial modulation systems. Str-AMP consists of two steps in each iteration. First, a decoupling operation is performed in the same way as AMP. Then, at the denoising step, to exploit the structured sparsity of SM signals, Str-AMP computes each element of SM signals using not only its own statistics but also those of its neighboring elements belonging to the same user. Since Str-AMP maintains scalar message passing, it is computationally efficient. Simulation results on various settings demonstrate the superiority of the proposed Str-AMP in terms of both uncoded and coded bit error rate performance.