A differential spatial modulation (DSM) with binary phase shift keying (BPSK) scheme is designed for two-way relay (TWR) wireless communications that use the denoise-and-forward (DNF) protocol without requiring channel state information at the sources and relay. The key area to make this scheme work is a multiple-to-one mapping method for DSM at the relay, as well as the DSM matrix design at the sources, which are proposed and derived in this letter. Simulation results show that the proposed system achieves significant gains in BER performance over traditional differential modulation with BPSK in DNF-TWR systems when both operate at the same spectral efficiency.