Transition metal particles dispersed on oxide supports are used as heterogeneous catalysts in numerous applications. One example is platinum clusters supported on ceria which is used in automotive catalysis. Although control at the nm-scale is desirable to open new technological possibilities, there is limited knowledge both experimentally and theoretically regarding the geometrical structure and stability of sub-nanometer platinum clusters supported on ceria. Here we report a systematic, Density Functional Theory (DFT) study on the growth trends of CeO2(111) supported PtN clusters (N = 1-10). Using a global optimization methodology as a guidance tool to locate putative global minima, our results show a clear preference for 2D planar structures up to size Pt8. It is followed by a structural transition to 3D configurations at larger sizes. This remarkable trend is explained by the subtle competition between the formation of strong Pt-O bonds and the cluster internal Pt-Pt bonds. Our calculations show that the reducibility of CeO2(111) provides a mechanism to anchor PtN clusters where they become oxidized in a two-way charge transfer mechanism: (a) an oxidation process, where Osurface atoms withdraw charge from Pt atoms forming Pt-O bonds, (b) surface Ce4+ atoms are reduced, leading to Ce3+. The active role of the CeO2(111) support in modifying the structural and eventually the chemical properties of sub-nanometer PtN clusters is computationally demonstrated.