The generation and matrix stabilization of ligand-free, small platinum nanoclusters (NCs) Pt12±x is presented. The metal-organic framework-template approach is based on encapsulating CO-ligated, atom-precise Pt9 Chini clusters [{Pt3(CO)6}3]2- into the zeolitic imidazolate framework ZIF-8. The selective formation of the air-stable inclusion compound [NBu4]2[{Pt3(CO)6}4]@ZIF-8 of defined atomicity Pt12 and with Pt loadings of 1-20 wt % was monitored by UV/vis and IR spectroscopy and was confirmed by high-resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD). Thermally induced decarbonylation at 200 °C yields the composite material Ptn@ZIF-8 with a cluster atomicity n close to 12, irrespective of the Pt loading. The PtNCs retain their size even during annealing at 300 °C for 24 h and during catalytic hydrogenation of 1-hexene at 25 °C in the liquid phase. The Ptn@ZIF-8 material can conveniently be used for storing small PtNCs and their further processing. Removal of the protective ZIF-8 matrix under acidic conditions and transfer of the PtNCs to carbon substrates yields defined aggregation to small Pt nanoparticles (1.14 ± 0.35 nm, HR-TEM), which have previously shown exceptional performance in the electrocatalytic oxygen reduction reaction (ORR).