In this paper, the sphere decoding (SD) detector for the generalized space-time shift keying (G-STSK) scheme is developed to reduce the computational complexity of the optimal detector. The proposed SD detector consists of a series of SD algorithms with each one associated with an activated dispersion matrix pattern. Specifically, first, a lower bound of the distance measure for each possible activated dispersion matrix pattern is proposed. Then, SD algorithms are performed successively after rearranging these patterns in the order of non-decreasing lower bound. Furthermore, the list SD detector is also presented which can be employed as the soft detector in the turbo receiver for the coded G-STSK scheme. Simulation results are provided to corroborate the proposed detectors.