Large single crystals of LiGaSe2 were grown and their spectroscopic properties were studied. As-grown LiGaSe2 crystals are transparent in the 0.5–14 μm spectral region as well as at λ > 200 μm in the THz domain. A large blue shift of the fundamental absorption edge was established after high-temperature annealing in the presence of Li. Intense low-temperature photoluminescence (PL) in the 1.77 eV band is excited in the 3.0–3.6 eV absorption band near the fundamental absorption edge; PL intensity decreases by two orders of magnitude as temperature grows to 300 K. The X-ray irradiation results in crystal darkening, whereas the original state is restored after heating to 500 K or by visible light illumination. This effect is due to six traps that capture charge carriers: these traps were revealed using the thermoluminesence technique and their parameters were calculated. Pyroelectric luminescence in the 100–270 K range confirms the noncentrosymmetric structure of this crystal. DFT calculations were carried out to simulate the phonon density of states, optical absorption and Raman spectra. Calculations show that the intense 3.0–3.6 eV absorption band and corresponding 1.77 eV PL band may be associated with the cation antisite defects (GaLi), i.e. gallium in the Li site. The 2.15 and 2.4 eV emissions are likely due to the recombination of self-trapped excitons.