A transparent NaBa₄Al₂B₈O₁₈Cl₃ (NBAC) crystal with dimensions of 38 × 36 × 10 mm³ was successfully grown by the top-seeded solution growth method using a new flux NaF–NaCl. The crystallization region of NBAC is revealed in a quasi-ternary system of NBAC–NaF–NaCl. By means of eliminating the extrinsic UV absorption of Fe³⁺ impurity, the UV absorption edge was redetermined to be 183 nm rather than the previously reported 230 nm. The nonlinear optical (NLO) coefficients of NBAC were measured by the Maker fringe technique, which gave results of d₁₅ = 1.27d₃₆(KDP) = 0.49 pm V⁻¹; d₃₃ = 2.25d₃₆(KDP) = 0.88 pm V⁻¹. Calculations on the local dipole moment reveal that the NLO effect of NBAC is attributable to ClNaBa₄ polyhedra, BO₃ triangles, and ClBa₄ tetrahedra. In addition, the ac dielectric and conductivity were measured using complex I–V method.