A clear description of the morphology and location, with respect to the support, of metallic subnanometric particles remains a current strenuous experimental challenge in numerous catalytic applications such as naphtha reforming and biomass conversion. High-resolution HAADF-STEM coupled with in situ and tomographic analyses have been undertaken on a platinum (Pt) active phase supported on chlorinated alumina (γ-Al2O3) with 0.3 and 1 wt % Pt loadings, highlighting the formation of flat nanoparticles (NPs) of 0.9 nm diameter and Pt single atoms (SAs) in the reduced state. While SAs and weakly cohesive clusters are predominantly observed in the oxide state, with a coordination sphere of Pt composed of O and Cl as revealed by EXAFS, the ratio between SAs and Pt NPs in the reduced state is found to be about 2.8. This ratio is the same for the two metal loadings: both the total numbers of NPs and SAs increase at a higher metal loading. Electron tomography reveals that the vast majority of NPs are located on the edges or defects (steps, kinks) of the γ-alumina support crystallites. DFT calculations further highlight the optimized structures of NPs located at the γ-Al2O3 (110)–(100) edge and near-edge with a stability competing with NPs located either on the (110) or on the (100) γ-Al2O3 facet. A mathematical analysis of the segmented volumes shows that the average geodesic distances between NPs is linked to Pt loading: 9 nm for 1 wt % Pt and 16 nm for 0.3 wt % Pt. Evaluation of support tortuosity descriptors using the nanoparticle positions confirms a uniform distribution on the support. A square network geometric model compatible with the geodesic distances between NPs reveals that one to five NPs can be present at the same time on each alumina crystallite depending on Pt loading.