A robust direct power control (DPC) approach is presented in this paper for three-phase grid-connected photovoltaic (PV) systems to control active and reactive power injection through voltage source inverters (VSIs). The dynamical model of three-phase grid-connected PV systems is considered based on the dynamics of active power, reactive power, and DC-link voltage rather than existing dq-axes current dynamics. Partial feedback linearization (FBL) technique is used to linearize the dynamical model of three-phase grid-connected PV systems as it is independent of operating points while simplifying the controller design process. The parameters in the feedback linearized system are considered as unknown and an adaptive backstepping approach is used to obtain the control law while the adaption laws are used to estimate the unknown parameters. The proposed controller will eliminate the parameter sensitivity problems of existing partial feedback linearizing controllers. The theoretical stability of the whole system as well as the convergences of physical properties during the estimation of unknown parameters are guaranteed through the negative definiteness or semi-definiteness of control Lyapunov functions (CLFs). Simulations are carried out to validate the effectiveness of the proposed controller and the results clearly confirm superior performance of the proposed partial FBL scheme as compared to a sliding mode controller (SMC) which has inbuilt robustness against parametric uncertainties.