Understanding how defect chemistry of oxide material influences the thermal stability of noble metal dopant ions plays an important role in designing high-performance heterogeneous catalytic systems. Here we use in-situ ambient-pressure X-ray photoemission spectroscopy (APXPS) to experimentally determine the role of grain boundary in the thermal stability of platinum doped cerium oxide (Pt/CeO₂). The grain boundaries were introduced in Pt/CeO₂ thin films by pulsed laser deposition without significantly change of the surface microstructure. The defect level was tuned by the strain field obtained using a highly/low mismatched substrate. The Pt/CeO₂ thin film models having well defined crystallographic properties but different grain boundary structural defect levels provide an ideal platform for exploring the evolution of Pt—O—Ce bond with changing the temperature in reducing conditions. We have direct demonstration and explanation of the role of Ce³⁺ induced by grain boundaries in enhancing Pt²⁺ stability. We observe that the Pt²⁺—O—Ce³⁺ bond provides an ideal coordinated site for anchoring of Pt²⁺ ions and limits the further formation of oxygen vacancies during the reduction with H₂. Our findings demonstrate the importance of grain boundary in the atomic-scale design of thermally stable catalytic active sites.