This paper presents an improved sliding mode-based finite control set model predictive controller for grid-tied single-phase multilevel inverter systems with an LCL filter. The proposed controller employs the inherent robustness of sliding mode control (SMC) with the fast dynamic response of the FCS-MPC method for controlling the complex behavior of LCL filter-based grid-tied inverter systems. The LCL-based grid-tied multilevel inverters necessitate the control of inverter side and grid side currents in addition to the grid filter capacitor voltage and DC-link capacitors voltage control/balance. Incorporating FCS-MPC with SMC eliminates the chattering effects in the traditional SMC algorithms. Moreover, incorporating SMC with FCS-MPC enables more robust and stable control performance compared to the conventional FCS-MPC method. The proposed method eliminates the need for weighting factors for DC-link capacitor voltage balance by properly selecting redundant voltage vectors. Simulation results are provided to validate the proposed SMC-based FCS-MPC controller with different active and reactive power injections to the grid.