Nanoparticle scintillators have been used in biological applications as internal light sources in biological bodies upon irradiation from outside. In such applications, different kinds of photoreceptor molecules having optical absorption in different wavelength regions are used. To be used in a noninvasive manner, nanoparticles are appropriate. Also, to achieve high stopping power for external irradiation of, e.g., X-rays, high density and effective atomic number are desirable. In this study, to fulfill these requirements, we developed nanoparticle scintillators based on Gd3Al3Ga2O12 (GAGG). To achieve emission in the yellow-to-red wavelength region, we doped Pr3+ ions in GAGG. We successfully developed GAGG scintillator nanoparticles of less than 100 nm that exhibit photoluminescence (PL) and scintillation at the yellow-to-red wavelength region owing to the 4f–4f transitions of Pr3+ ions. The PL quantum yield of 62% was achieved at the Pr concentration of 1 mol%.