Metal oxyhalides are attracting extensive interest for their enchanting structures and diverse properties. Herein, a ternary antimony oxyiodide, Sb5O7I with the new hexagonal noncentrosymmetric P63 structure is systematically surveyed by focusing on its nonlinear-optical (NLO) behavior. Its two-dimensional structure is constructed by {Sb2[Sb3O7]}∞+ layers separated by charge-balanced I- anions. The second-harmonic-generation measurement result suggests that Sb5O7I is NLO-active, and the effect is assigned to the [SbO3]3- triangular pyramids' contribution. Sb5O7I shows a direct optical energy gap of 3.22 eV, which is the largest among all reported ternary oxyiodides. This work is the first investigation of ternary NLO Sb-based oxyhalides and enriches the study of metal oxyhalides as promising NLO materials.