Grid-connected converters (GCC) commonly rely on proportional–integral (PI) controllers for DC-link voltage regulation. However, their performance can significantly degrade in the presence of parameter variations and external disturbances, resulting in overshoot, undershoot, and slow transient response. To address these limitations, this paper proposes a generalized proportional–integral observer (GPIO)-based control strategy for single-phase GCC. The proposed method improves disturbance estimation and rejection, thereby enhancing robustness against system uncertainties while ensuring fast and accurate DC-link voltage regulation. The main contribution of this work lies in the development of a GPIO-based controller that combines improved dynamic performance with strong disturbance rejection capability, without increasing control complexity. The proposed approach is validated through both simulation and experimental results, which demonstrate its effectiveness and superiority over conventional PI-based control in terms of transient response, stability, and robustness.