We develop two new designs of unitary differential space–time modulation (DSTM) with a low decoding complexity. Their decoder can be separated into a few parallel decoders, each of which having a decoding search space of less than if the DSTM codebook contains codewords. Both designs are based on the concept of joint modulation, which means that several information symbols are jointly modulated, unlike the conventional symbol-by-symbol modulation. The first design is based on an orthogonal space–time block code (O-STBC) with joint constellation constructed from spherical codes. The second design is based on a quasi-orthogonal STBC (QO-STBC) with specially designed pairwise constellation sets. Both the proposed unitary DSTM schemes have a considerably lower decoding complexity than many prior DSTM schemes, including those based on group codes and Symplectic group Sp(2), which generally have a decoding search space of for a codebook size of codewords and a much better decoding performance than the existing O-STBC DSTM scheme. Between the two designs, the proposed DSTM based on the O-STBC generally has a better decoding performance, whereas the proposed DSTM based on the QO-STBC has a lower decoding complexity for eight transmit antennas.