A new thiogallate-based green-emitting phosphor, MgGa2S4:Eu2+, was first synthesized via a high-temperature solid-state reaction in a CS2 atmosphere. We then investigated the structures and luminescent properties of the MgGa2S4:Eu2+ phosphors. The MgGa2S4:Eu2+ phosphors can be excited efficiently by UV–visible light in the wavelength range from 350 to 520 nm and they emit an intensely green light with emission bands peaking at 538 nm. The optimal concentration for Eu2+ in MgGa2S4 was found to be about 6 mol%, and the corresponding concentration quenching mechanism was the electric multipole–multipole interaction. The quenching temperature was calculated to be 402 K, and the Huang–Rhys factor was about 4. The energy barrier for thermal quenching was calculated to be 0.28 and 0.27 eV by the two types of the Arrhenius equations. The small variation in the color coordinates of MgGa2S4:Eu2+ under high temperatures indicates that the as-synthesized phosphor has good color stability. Due to their broadband absorption in the 350–520 nm wavelength range, these phosphors may be able to fulfill the requirements for application in the development of Ga(In)N-based white LEDs.