This paper investigates fading correlation in space-diversity free-space optical (FSO) communication systems and its effect on the link performance. We firstly evaluate the fading correlation in multiple-aperture FSO systems using wave-optics simulations. The influence of different system parameters including the link distance and aperture spacing is illustrated under realistic beam propagation conditions. In particular, we show that, at relatively large link distances where the scattering disk is much larger than the receiver aperture size, the fading correlation coefficient is almost independent of the apertures' diameter and depends only on the apertures' edge separation. To investigate the impact of fading correlation on the system's performance, we propose an analytical approach to evaluate the performance of the space-diversity FSO system over a correlated Gamma–Gamma (ΓΓ) fading channel. Our approach is based on approximating the sum of arbitrarily correlated ΓΓ random variables by an α−μ distribution. To validate the accuracy of this method, we evaluate the average bit-error-rate (BER) performance for the case of a multiple-aperture FSO system and compare it with the BER results obtained via Monte Carlo simulations.