A novel microwave-assisted synthesis technique was used for the rapid preparation of nanocrystalline ZnGa2O4 at two different temperatures. The crystalline spinel oxide is formed at temperatures as low as 100
C within few minutes, at a high yield of 96%, requiring no post-synthesis annealing. The as-prepared samples are polycrystalline and phase-pure as verified by XRD, with a crystallite size of ∼5 nm. Polycrystalline ZnGa2O4 substituted with Mn2+, Cr3+, Cu2+, and Co2+ was also similarly prepared. All samples are highly monodispersed, as measured by TEM. The ZnGa2O4 nanocrystals without further surface modification can be readily dispersed in chloroform to form a fully transparent colloidal solution, using which the bandgap of ZnGa2O4 was determined to be ~4.5 eV. The entire synthesis procedure, including solution preparation, microwave irradiation, and centrifugation takes about 30 minutes, which is faster than any procedure reported for a complex oxide like ZnGa2O4, as well as one with a small thermal budget. Photoluminescence shows a broad emission extending from 330 nm to 800 nm, which is surmised to be due to the defect structure in the oxide produced.