Overexpression of HER2 significantly reduced sensitivity to sotorasib in KRAS-G12C models but not in KRAS-G12S mutant A549 cells. In resistant cells, MAPK signaling remained active despite sotorasib treatment. Combining a SHP2 inhibitor (TNO155) and sotorasib synergistically overcame HER2-mediated resistance in vitro and in vivo. Our findings introduce HER2 gain as a clinically relevant off-target resistance mechanism to KRAS-G12C inhibition that can be rescued by co-targeting SHP2.