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The elaboration and study of hybrid nanocomposites based on photoswitchable spin-crossover nanoparticles is reported. A silica polymeric gel is used as the confining medium to control the kinetics of nucleation and growth of a molecular spin-crossover prototype [Fe((mepy)3tren)](PF6)2. The precipitation of nanoparticles in the matrix is triggered by spin-coating of the doped gel on a convenient substrate. This process leads to spherical particles of controlled size from 730 (± 80) to 47 (± 10) nm homogeneously dispersed in transparent silica thin films. The chemical integrity of the coordination compound is checked by EDS and Raman spectroscopies. UV–Vis measurements confirm the persistence of a spin-crossover regime for these nanocomposites. Indeed, the MLCT absorption features are typical of the molecules being in the high-spin state at high temperature and in the low-spin state at low temperature. With respect to the microcrystalline parent compound, the spin-crossover curves afforded by the nanoparticles do not significantly vary over the explored size range. In addition, they are strongly shifted toward lower temperatures. This feature is accounted for by the in-silica formation of a quenched product which behaves like a new phase generated by sudden precipitation of [Fe((mepy)3tren)](PF6)2. Indeed the precipitated bulk phase, characterized by powder XRD, exhibits both magnetic and optical characteristics very close to those of the nanoparticles. The photoswitching properties based on light-induced excited spin-state trapping (LIESST) are probed by UV–Vis and magnetic measurements. The complexes embedded in silica thin films can be efficiently photoexcited and evidences are provided for the formation of a metastable HS state. [ABSTRACT FROM AUTHOR]