Abstract: Although Sn has been established as an effective microalloying element for suppressing the negative natural aging (NA) effect in Al–Mg–Si alloys, its potential to mitigate the negative NA effect in Al–Mg–Si–Cu alloys remains to be confirmed. This study systematically investigated the role of Sn in the NA of Al–Mg–Si–Cu alloys through hardness measurements, differential scanning calorimetry, and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. Our results demonstrate that the addition of Sn significantly suppresses the adverse impact of NA on the peak-aged hardening capacity during subsequent artificial aging and substantially alleviates early-stage hardening kinetics degradation. Our findings suggest that Sn modifies the nature of the NA clusters in Al–Mg–Si–Cu alloys. A significant proportion of NA clusters in the Sn-added alloy effectively served as heterogeneous nucleation sites for strengthening the precipitates during artificial aging, thereby preserving the precipitate nucleation rates and preventing coarsening at the peak-aging stage. Atomic-resolution energy-dispersive X-ray spectroscopy revealed preferential occupation of Si atomic sites by Sn atoms within the β′ and C/Q′ phases. This investigation provides critical theoretical insights for optimizing alloy design in automotive-body aluminum applications.