Gallium(II) sulfide, GaS, is a chemical compound of gallium and sulfur. The normal form of gallium(II) sulfide as made from the elements has a hexagonal layer structure containing Ga<sub>2</sub><sup>4+</sup> units which have a Ga-Ga distance of 248pm. This layer structure is similar to GaTe, GaSe and InSe. An unusual metastable form, with a distorted wurtzite structure has been reported as being produced using MOCVD. The metal organic precursors were di-tert-butyl gallium dithiocarbamates, for example Ga<sup>t</sup>Bu<sub>2</sub>(S<sub>2</sub>CNMe<sub>2</sub>) and this was deposited onto GaAs. The structure of the GaS produced in this way is presumably Ga<sup>2+</sup> S<sup>2<nowiki>−</nowiki></sup>.
Single layers of gallium sulfide are dynamically stable two-dimensional semiconductors, in which the valence band has an inverted Mexican-hat shape, leading to a Lifshitz transition as the hole-doping is increased.
Gallium(II) sulfide demonstrates nonlinear optical activities, including second-harmonic generation and two-photon excited fluorescence.