The low‐temperature photoluminescence spectra of AgGaS2are investigated in this work. At 4.2°K, the luminescence spectra are dominated by many sharp lines as well as broad emission bands. A series of emission lines was found to be superimposed on the broad band peaking at ∼ 5000 Å. The series of emission lines are attributed to phonon‐assisted transitions involving an unidentified impurity or defect. Two basic phonons with energies of 5.4 and 7.2 meV are identified from the phonon structure. The 7.2‐meV mode is also seen in the Raman spectrum. They are tentatively identified as the lattice modes.