In this paper, a robust adaptive Sliding Mode Controller (SMC) with an optimized Proportional-IntegralDerivative (PID) sliding surface has been proposed to control the voltage and frequency of standalone inverter-interfaced renewable energy resources. The suggested controller improves the tracking performance in order to tend the tracking error to zero, and also rejects the uncertainties and disturbances. The adaption law, which is derived from the Lyapunov theory, improves the controller performance under uncertainties and unknown parameters, such as variations in input DC link and different kinds of loads (linear and non-linear loads). Also, the chattering issue in first-order SMC is eliminated by using the second-order SMC with a PID sliding surface. In addition, an improved particle swarm optimization algorithm has been used to optimize and extract the controller variables. The suggested controller improves the total harmonic disturbances, output voltage peak, root-mean-square, and tracking error in comparison to the other controllers, such as SMC and backstepping controller. The efficiency of the proposed controller is verified by simulation in Matlab software package.