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A detailed physico-chemical study of Cr3+-doped ZnO nanoparticles (NPs) obtained via aqueous synthesis was carried out. The NPs acquire greenish tonality as the Cr concentration increased. High efficiency of Cr incorporation into the ZnO lattice was found up to approximately 1 at% Cr. The ZnO:Cr NPs show wurtzite hexagonal phase with a lattice compressed for low Cr concentration and tensed for higher concentrations. Structural, chemical and optical analyses indicate that for low Cr concentration Cr occupies the Zn vacancy sites, owing to the reduction in acceptor compensation by passivating the Zn vacancies and the contribution of the unpaired electron of Cr3+, which induces an increase of electron concentration, changes in the conductivity, transport of carriers, optical properties and ferromagnetic properties, among others. At higher Cr concentrations, the passivation of Zn vacancies by Cr atoms reduces the acceptor contribution, and substantial substitution of Zn atoms by Cr atoms in the ZnO lattice strengthens the retention of O atoms in the Cr-O bonds. This results in a reduction in O vacancy concentration, donor contribution, strain, electron concentration, and luminescence intensity.