In search of new nonlinear optical crystals for the mid-IR range, we grew single crystals of LiGa 0.55 In 0.45 Te 2 (LGITe) of optical quality; its structure was established, and the spectroscopic properties were studied. LGITe crystallizes in a tetragonal chalcopyrite-like (I42d) structure at 667 ± 5 °C. Crystals are transparent in the 0.76-14.8 μm spectral region with some absorption bands related to OH/H 2 O and Ge-O vibrations. Analysis of the Tauc plots showed that LGITe is a direct band gap semiconductor with the band gap E g = 1.837 eV at 300 K. The Raman spectrum for LGITe is very similar to that for LiGaTe 2 : the most intense band A 1 near 120 cm -1 corresponds to the Li-Te vibration. DFT calculations were carried out for the first time to simulate the energy band structure (band gap 1.67 eV), density of states (DOS), birefringence (0.049 at 2 μm), and nonlinear susceptibility (d 14 = d 36 = 48.73 pm/V). For ternary LiInTe 2 these parameters are n = 0.007 and d 14 = d 36 = 61.4 pm/V, respectively. The calculated optical properties indicate that LiGa 0.55 In 0.45 Te 2 is a promising mid-IR nonlinear optical crystal with nonlinear parameters higher than those of LiGaTe 2 .