In this paper, we propose two minimum mean square error (MMSE)-based multiuser detectors, called the combined MMSE and maximum likelihood (MMSE-ML) multiuser detector and the combined MMSE and successive interference cancellation (MMSE-SIC) multiuser detector, for an uplink space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) system. Both detectors provide significantly improved bit error rate (BER) over the conventional MMSE multiuser detector while requiring the same complexity order. Moreover, the combined MMSE-ML detector exhibits better BER than the combined MMSE-SIC, particularly, in a system with a small number of users.