Two-dimensional (2D) materials have aroused tremendous interest due to their great potential applications in electronic, optical, and mechanical devices. We theoretically design a new 2D material SiGeS by regularly arranging the Si-S-Ge skeleton of SiH₃SGeH₃. Based on first-principles calculation, the structure, stability, electronic properties, mechanical properties, and optical properties of SiGeS are systematically investigated. Monolayer SiGeS is found to be energetically, dynamically, and thermally stable. Remarkably, the SiGeS displays a unique negative Poisson’s ratio. Besides, the SiGeS is an indirect-semiconductor with a band gap of 1.95 eV. The band gap can be modulated effectively by applying external strains. An indirect-to-direct band gap transition can be observed when the tensile strain along the x axial or biaxial direction is greater than +3%, which is highly desirable for applications in optical and semiconductor technology. Moreover, pristine SiGeS has a high absorption coefficient (～10⁵ cm^–1) in a visible-to-ultraviolet region. Under tensile strain along the x axial direction, the absorption edge of SiGeS has a red shift, which makes it cover the whole region of solar spectrum. These intriguing properties make the SiGeS a competitive multifunctional material for nanomechanic and optoelectronic applications.