The nonstoichiometry of I−III−VI semiconductor nanoparticles, especially the ratio of group I to group III elements, has been utilized to control their physicochemical properties. We report the solution-phase synthesis of nonstoichiometric Ag−In−S and Ag−In−Ga−S nanoparticles and results of the investigation of their photoluminescence (PL) properties in relation to their chemical compositions. While stoichiometric AgInS 2 nanoparticles simply exhibited only a broad PL band originating from defect sites in the particles, a narrow band edge PL peak newly appeared with a decrease in the Ag fraction in the nonstoichiometric Ag−In−S nanoparticles. The relative PL intensity of this band edge emission with respect to the defect-site emission was optimal at a Ag/(Ag + In) value of ca. 0.4. The peak wavelength of the band edge emission was tunable from 610 to 500 nm by increased doping with Ga 3+ into Ag−In−S nanoparticles due to an increase of the energy gap. Furthermore, surface coating of Ga 3+ -doped Ag−In−S nanoparticles, that is, Ag−In−Ga−S nanoparticles, with a GaS x shell drastically and selectively suppressed the broad defect-site PL peak and, at the same time, led to an increase in the PL quantum yield (QY) of the band edge emission peak. The optimal PL QY was 28% for Ag−In−Ga−S@GaS x core−shell particles, with green band-edge emission at 530 nm and a full width at half-maximum of 181 meV (41 nm). The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic-element-free I−III−VI-based nanoparticles in a wide area of applications.