The title compound, Ba4Ga2B8O18Cl2, NaCl, is found to crystallize in a polar space group P4(2)nm with cell dimensions of a = 12.1134(2) angstrom and c = 6.8456(1) angstrom. The basic building blocks of the structure are the B4O9 groups, which are interconnected by the GaO4 tetrahedron to form a three-dimensional net with Ba-2(+) ion-, Cl-ion-, and NaCl molecule-filled tunnels. This net structure is closely related to that of mineral hilgardite, with which many variant compounds have been found. Both a powder second-harmonic-generation test and calculations suggest that it possesses an optical nonlinearity comparable to that of potassium dihydrogen phosphate.