A differential phase-shift keying modulation for free-space optical (FSO) communication is considered in atmospheric turbulence modeled by the exponentiated Weibull distribution. The selection combining (SelC) spatial diversity is used to mitigate the effects of atmospheric turbulence. We analyze the average bit error rate (BER) of the system using SelC spatial diversity by Gauss–Laguerre approximation. The effect of aperture averaging and spatial diversity on the outage probability is also studied. The numerical results show that it requires a smaller level of signal-to-noise ratio to reach the same BER when large aperture and SelC spatial diversity are deployed in the FSO system. Moreover, it is proved that aperture averaging and SelC spatial diversity are effective for improving the performance of the system’s outage probability.