<p>The first-principles calculations are employed to investigate the segregation and diffusion behavior of Pb at γ-Fe grain boundaries (GBs). Pb shows a strong tendency to segregate at open-structured GBs, whereas the coherent twin boundary exhibits exceptional resistance to Pb segregation. The thermodynamic segregation tendency of Pb is influenced by temperature, the extent of matrix corrosion, and GB type. A novel method is proposed to predict the segregation tendency, revealing a linear relationship between segregation energy and cumulative interstitial volume at substitutional sites. This approach offers a new perspective for forecasting the segregation behavior of oversized solutes. Furthermore, Pb diffusion is found to be significantly faster along GBs than in the bulk, with especially high mobility in open-structured GBs that possess a high coherency density. These atomic-scale insights establish a foundation for fundamentally understanding intergranular corrosion by liquid Pb and offer a theoretical basis for grain boundary engineering strategies to mitigate liquid Pb corrosion in advanced structural materials.</p>