Over a correlated flat fading channel, multiple-symbol differential detection can enhance the performance of coded differential phase shift keying (DPSK) systems but with exponential complexity. For iterative decoding schemes, the soft-input soft-output (SISO) multiple-symbol differential sphere decoding (MSDSD) can offer suboptimal performance and its complexity is quadratic with detection length. To further reduce the complexity, this paper proposes a Forward/Backward MSDSD (FB-MSDSD) for coded DPSK systems. The key idea is that the detection interval is split into two subintervals which are processed in the forward and backward directions respectively. Simulation results show that the proposed scheme has almost the same performance and lower complexity when compared with the SISO-MSDSD scheme with the same detection length.