Free-space optics (FSO) systems use the atmosphere as a medium to propagate data in the form of an optical signal. Using mode division multiplexing (MDM) over FSO systems increases the capacity of the FSO link. However, atmospheric turbulence is one of the main problems that seriously limit the data transmission rate in FSO systems. This causes signal distortion during transmission that leads to data loss at the receiver side totally or partially. To address this problem, a decision feedback equalization (DFE) scheme with minimum mean square error (MMSE) in MDM for FSO systems was developed in OptSim 5.2 software to mitigate the adverse effects of atmospheric turbulence. The MMSE algorithm was used to optimize both the feed-forward and feedback filter coefficients of the DFE. The proposed DFE scheme in MDM for the FSO system comprises seven parallel 2.5 Gbps MDM channels using Hermite–Gaussian modes at the transmitter to transmit 17.5 Gbps and seven DFE schemes at the receiver to mitigate the effects of atmospheric turbulence. The simulation results demonstrate that the DFE scheme can transmit 17.5 Gbps over 40 m, 800 m, 1400 m, and 2 km under medium fog, medium rain, medium haze, and clear weather, respectively. We show that the DFE scheme improves the MDM over FSO system immunity to distortion in medium fog, medium rain, medium haze, and clear weather while maintaining high throughput and desired low bit error rate.