A new acentric sulfide YSiS was grown by a high temperature vapor transport reaction. The crystal structure of YSiS was determined by single crystal X-ray diffraction. YSiS is isostructural to LaGeS. The three dimensional structure of YSiS is constructed by [Y1S] augmented triangular prisms, [Y2S] triangular prisms and [SiS] tetrahedra through sharing vertices and edges. YSiS is revealed as an indirect bandgap semiconductor with a calculated bandgap of 2.1 eV, which is close to 2.5(1) eV experimentally measured by UV-Vis. The Y−S interactions and Si−S interactions are predicated to be strong ionic bonds and covalent bonds, respectively, by electron localization function coupled with crystal orbital Hamilton population calculations. YSiS is verified by DFT calculations to have moderate birefringence, with incident 1900 nm laser, Δn=0.09. DFT calculations also predicted that YSiS possesses moderate second harmonic generation response with χ=15.03 pm/V. The nonlinear optical properties of YSiS are mainly contributed by Y−S interactions revealed by DFT calculations.