This letter proposes a decoding scheme for non-orthogonal space-time block codes (NO-STBCs) in overloaded multiple-input multiple-output systems, where the number of different transmitted streams is greater than that of receive antennas. For the underdetermined decoding problem, we iteratively solve a convex optimization problem with the update of the parameters in the objective function, which utilizes the discreteness of the transmitted symbols. For the NO-STBC based on cyclic division algebra, we can reduce the order of computational complexity of the algorithm by taking advantage of the structure of the code. Simulation results show that the proposed algorithm outperforms conventional schemes in the decoding of large-scale overloaded NO-STBCs.