Recently, Si quantum dots (QDs) with porous and amorphous characteristics have shown tremendous potential applications in photovoltaics and biomedical fields. However, in general, their preparation involves meticulous synthetic procedures. Herein, we present the first report of the synthesis of Si QDs in aqueous solution with amorphous and porous features using γ-ray irradiation, which is a facile, one-pot, and reproducible approach. Fourier transform infrared and X-ray photoelectron spectroscopy indicated the formation of a shell of Si oxides (SiOx, 0 < x < 2) surrounding the Si core. The Brunauer−Emmett−Teller method confirmed the formation of mesoporous structure with a specific surface area of ∼53 m2/g. The average size of the QDs was <2 nm, as determined from high-resolution transmission electron microscopy. Moreover, the size of the QDs could be tuned by varying the dose, as was indicated from the shift in their absorption peaks. The QDs displayed bright blue emission under ultraviolet light. Detailed and systematic investigations involving various sets of experiments such as aging, annealing, acid treatment, and temperaturedependent PL studies of the QDs indicate the origin of the blue photoluminescence (PL) is the radiative processes originating from the defects located in the oxide shell as well as at/or near the Si/SiOx interface. This has been further substantiated from the size-independent emission spectra at various absorbed doses and short PL lifetime (in nanoseconds). Nonetheless, these QDs showed highly sensitive and robust thermosensing properties, as their PL intensity varied linearly and reproducibly with temperature, ranging from 278 to 338 K. In addition, the QDs were found to be noncytotoxic and therefore can be utilized in cell imaging. Furthermore, the blue PL and the nontoxic aspects of these QDs can be utilized in anticounterfeiting measures, the potential application of which is also demonstrated in this work. The possible mechanism behind the formation of Si QDs and the role of the total dose in the introduction of mesoporous and amorphous features are proposed and explained in this paper.