Spinel-type metal oxide nanoparticles are known to exhibit various properties, which can be further enhanced by the synthesis of nonstoichiometric nanoparticles. In this work, we report the production of nonstoichiometric cobalt gallate nanoparticles (Co−Ga NPs) with larger inversion parameters that resulted in enhanced magnetic properties compared to previous studies. The synthesis of nonstoichiometric nanoparticles was realized by a supercritical hydrothermal process and without performing calcination at a high temperature which tended to synthesize thermodynamically stable stoichiometric products. We synthesized nonstoichiometric Co−Ga NPs with a controllable Co/Ga molar ratio, morphology, and particle size at different pH values of the precursor solutions. By applying X-ray absorption fine structure analysis at Ga K-edge, Co K-edge, and Rietveld refinement, nonstoichiometric Co−Ga NPs were found to have large numbers of cobalt atoms occupying octahedral sites compared to CoGa 2 O 4 . Their superparamagnetic behavior with enhanced spontaneous magnetization at room temperature owing to percolated superexchange interaction of J BB was observed for the first time. We also proposed a possible formation mechanism of nonstoichiometric Co−Ga NPs from cobalt gallium nitrate layered double hydroxides in the precursor solutions.