The phosphor SrGa2S4:Sn exhibits a new green photoluminescence (PL) whose spectrum peaks at 534 nm. The phosphor also exhibits yellow and orange PL with very broad spectral bands when the excitation wavelength of the UV light is changed. These emissions are caused by the 5p–5s transition in the 5s2 configuration of Sn2+ ions occupying Sr2+ sites. The origin of the green emission was investigated by analyzing the PL excitation spectra and the electronic states of Sn2+ substituting at three different Sr2+ sites. The electronic states were analyzed in a first-principles calculation of the density of states.