The complete solid solution LixAg1–xAlSe2 was prepared at 850 °C in the form of phase-pure powder samples as well as single crystals. They adopt noncentrosymmetric crystal structures belonging to the tetragonal CuFeS2-type (space group I4‾2d) for x = 0–0.5 and orthorhombic β-NaFeO2-type (space group Pna21) for x = 0.6–1, both being built up from corner-sharing metal-centred tetrahedra and exhibiting complete disorder of Li and Ag atoms. Solid-state 7Li NMR spectra support the presence of unique Li sites. The substitution of Li for Ag atoms allows control of optical band gaps, which increase linearly from 2.7 eV to 3.6 eV. Second harmonic generation responses were measured to evaluate the potential of these compounds as infrared nonlinear optical materials.