Space time block codes (STBC) have become a popular transmit-diversity technique due to their simplicity and high gain under a frequency-flat-fading channel and so have been adopted for the 3G wideband code division multiple access system (W-CDMA). However, the performance of an STBC is degraded under a multiuser and frequency-selective fading environment. The paper presents an improvement in the STBC under such environments by spreading the information symbols with a complex pseudo-noise (PN) sequence, called a Song-Park (SP) sequence. The SP sequence has a good even and odd cross-correlation property so that the intersymbol interference (ISI) and multiuser access interference (MAI) can be almost negligible. Therefore, a significant result is that the performance of the proposed scheme under a synchronous or asynchronous multiuser environment is close to that of a single user. In addition, the proposed system is much simpler than known multiuser detection (MUD) schemes, such as MMSE and zero-forcing (ZF) schemes, because the proposed system simply spreads and despreads the information symbols with the SP sequence consisting of complex numbers on the unit circle. The paper also extends the proposed idea to a differential STBC (D-STBC) system because a D-STBC does not require any channel estimation. Again, the proposed D-STBC spread with an SP sequence shows superior performance over the ordinary D-STBC with no spreading or a D-STBC spread with another sequence, such as a Walsh orthogonal sequence.