Third-order optical nonlinearity (TONL) of chalcogenide glasses (ChGs) in Ge-In-S (GIS) ternary system is characterized at two near-infrared wavelengths of 0.8 μm and 1.064 μm. It is found that the GIS ChGs with non-stoichiometric compositions exhibit extremely large TONL properties, and its origin has been explained from both aspects of glass structure and wavelength-dispersion dependence of the TONL properties. Nevertheless, calculation of figure of merits indicates that the GIS ChGs are suitable for n2-based TONL optical devices at long wavelength, while suitable for β-based devices at short wavelength