A dual-hop decode-and-forward relay-assisted \(2\times 1\) multiple-input single-output free-space optical (FSO) communication system is considered. Error performance of the considered FSO system is analyzed with an arbitrary beamforming scheme in Gamma–Gamma distributed atmospheric turbulence and misalignment errors. The misalignment errors in the receive apertures of relay and destination are assumed to be zero-boresight, where the radial displacement in the receiver plane is modeled by Rayleigh distribution. The probability density function and moment generating function of received signal-to-noise ratio (SNR) for both the hops are derived first. By utilizing the derived statistics, the expressions of average symbol error rate for subcarrier intensity modulated-based M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) constellations are obtained in the destination. Monte-Carlo simulations are presented to support the analysis.