Grain boundaries (GBs) can be used as traps for solute atoms and defects, and the interaction between segregants and GBs is crucial for understanding the properties of nanocrystalline materials. In this study, we have systematically investigated the tendency of Pt to segregate as well as the interaction between Pt and oxygen vacancies at the sigma 3/(111) GB of ceria (CeO2). The Pt atom has a stronger tendency to segregate to the sigma 3/(111) GB than to the (111) and (110) free-standing surfaces, but the tendency is weaker than to the (100) free-standing surface. Mechanic contributions (lattice distortion) play a dominant role in the strong tendency of Pt to segregate. At the Pt-segregated-GB (Pt@GB), oxygen vacancies prefer to form spontaneously near Pt in the GB region. However, at the pristine GB (no Pt and no vacancies), oxygen vacancies form only under O-poor conditions. Thus, Pt segregation to the GB promotes the formation of oxygen vacancies, and their strong interactions enhance the interfacial cohesion. We propose that GBs fabricated close to the surfaces of nanocrystalline ceria can trap Pt from the bulk or other types of surface, resulting in the suppression of the accumulation of Pt on the surface under redox reactions, especially under O-poor conditions.