In this paper, we propose and study a novel Time—frequency-domain iterative multiuser detector (TF-IMUD) for the multicarrier direct-sequence code-division multiple-access (DS-CDMA) systems employing both time (T)-domain and frequency (F)-domain spreading, which, for brevity, are referred to as the TF/MC DS-CDMA systems. This novel TF-IMUD consists of a set of T-domain soft-input soft-output MUDs (SISO-MUDs) and a set of F-domain SISO-MUDs, where information is exchanged between the T-domain and F-domain through two multiuser interference (MUI) cancellation units. The T-domain and F-domain SISO-MUDs are operated under the maximum likelihood (ML) principles, and they are facilitated to be implemented using parallel signal processing techniques. In this paper, the complexity of the TF-IMUD and the bit error rate (BER) performance of the TF/MC DS-CDMA systems employing the proposed TF-IMUD are investigated and also compared with the other existing MUD schemes, including the optimum ML-MUD and the joint/separate minimum mean-square error (MMSE)-MUDs. Our study and simulation results show that the TF-IMUD is capable of achieving nearly the same BER performance as the optimum ML-MUD, but at significantly lower complexity. Copyright © 2010 John Wiley & Sons, Ltd.