Intense search for new nonlinear optical crystals for the mid-infrared region is in progress, and Li-containing quaternary chalcogenides are expected to improve transparency range, stability, phase-matching conditions, and other parameters in comparison with commercially available AgGaS2, AgGaSe2, and ZnGeP2. Single crystals of Li2In2GeSe6 up to 8 mm in size were obtained by the Bridgman-Stockbarger growth technique, and their high quality was confirmed by exciton luminescence. A monoclinic structure and direct band-to-band electronic transitions were established, and the thermal expansion was shown to be virtually isotropic. Defect-related absorption and luminescence were revealed, and the way to lower them was suggested. The electronic structure, density of states, and some optical properties were calculated from the first principles for Li2In2GeSe6. The calculated nonlinear coefficients and rather large birefringence indicate a strong phase-matching ability. These investigations demonstrate that Li2In2GeSe6 is a promising mid-infrared nonlinear optical crystal.