Compact CeO2(111) films grown on Ru(0001) can be transformed into well-shaped nanoparticles by annealing them in an oxygen-poor environment. With increasing temperature, the particles undergo a distinct shape evolution that finally leads to crystallites exposing wide (100) facets. The atomic structure of the (100) termination is determined with a combination of high-resolution scanning tunneling microscopy and density functional theory. Two surface reconstructions are identified that are compatible with the need to compensate for the intrinsic dipole of the (100) plane and with a substantial reduction of the oxide material. Our study provides insights into the rarely explored (100) surface of ceria, which can be considered as model system for studying chemical processes on the polar termination of reducible oxides.