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The discovery of a photomagnetic effect in a CoFe Prussian blue analog (PBA) has triggered a growing interest for photo‐switchable bimetallic cyanide‐bridged systems. Nevertheless, in between cyanide‐bridged extended coordination polymers and discrete molecules, the photo‐switching phenomena are much less well known in nano‐sized materials. A photo‐induced transformation, specific to the nanometric size, is evidenced by magnetometry and by X‐ray absorption spectroscopy at the Co and Fe K‐edges in an alkali cation free Prussian blue analog. The nanoparticles before irradiation can be described as having a core‐shell structure, the core being made of the well‐known fcc‐CoII(HS)FeIII structure of CoFe PBAs while the shell contains CoII ions in octahedral geometry and significantly distorted Fe(CN)6 entities. Irradiation induces a change of the local structures around the transition metal ions, which remain in the same oxidation state, with different behaviors of the Co and Fe sub‐lattices.