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Structural modulations have been recently found to cause some unusual physical properties, such as superconductivity or charge density waves; however, thus-induced nonlinear optical properties are rare. We report herein two unprecedented incommensurately modulated nonlinear optical sulfides exhibiting phase matching behavior, A 2 SnS 5 (A=Ba, Sr), with the (3+1)D superspace groups P2 1 2 1 2(00γ)00s or P2 1 (α0γ)0, featuring different modulations of the [Sn 2 S 7 ] ∞ belts. Remarkably, Ba 2 SnS 5 exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS 2 at 1570 nm and a very large laser-induced damage threshold (LIDT) of 8×AgGaS 2 . Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A 2 SnS 5 (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α-Ba 2 SnSe 5 without modulation. Besides, despite the smaller E g , the A 2 SnS 5 samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba 2 SnS 5  (1.51)2 SnS 5  (2.08)2  (2.97))., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)