We report the phase-transparent optical data exchange of differential phase-shift keying (DPSK) signals by exploiting the parametric depletion of nondegenerate four-wave mixing (FWM) in a highly nonlinear fiber. Theoretical analyses of nondegenerate FWM involving two signals and two pumps are presented. Analytical solutions are derived, indicating the exchange condition and the feasibility of phase-transparent data exchange. Optical data exchange between $10.7\mathrm{Gbit}/\mathrm{s}$ non-return-to-zero DPSK (NRZ-DPSK) and return-to-zero DPSK signals is implemented with a power penalty of ~1.5 dB at a bit-error rate (BER) of ${10}^{-9}$ . Moreover, we demonstrate phase-transparent optical data exchange between two $40\mathrm{Gbit}/\mathrm{s}$ NRZ-DPSK signals with a power penalty of ~4.5 dB at a BER of ${10}^{-9}$ .