Regarding light absorption/emission efficiency, silicon presents the fundamental drawback of its indirect band gap. It is long known, though, that optical properties are greatly enhanced in materials which comprise different kinds of nanocrystalline Si covered by or embedded in Si oxide layers. Conversely, their amorphous counterparts have received far less attention, such that no general consensus about the emission mechanism prevails. We report here on an efficiently luminescent material based on amorphous Si nanoparticles (a-Si NPs) embedded in a nonstoichiometric Si oxide matrix, which exhibits intense, broadband emission from the a-Si NPs, spectrally separable from the defect luminescence of the suboxide matrix. Apart from the brightness of the emitted light, the nanometer-size a-Si inclusions present the technological advantage of needing very moderate annealing temperatures (450∘C–700∘C) for their production. The combined use of high pressure, experimentally as well as theoretically, allowed us to trace back the microscopic origin of the photoluminescence to radiative recombination processes between confined states of the a-Si NPs. The signature of quantum confinement is found in the magnitude and sign of the pressure coefficient of different optical transition energies. The pressure derivatives exhibit a universal dependence on particle size, determined solely by the confinement energy of the discrete electron state involved in the radiative recombination process.