We report the synthesis of ultrathin indium sulfide In₂S₃ nanoribbons which display a giant aspect ratio using a simple and fast solvothermal method. They have a sub-nanometer thickness controlled at the atomic level, a width of (8.7 ± 0.1) nm and a length which can reach several micrometers. We determine the atomic composition of the inorganic core by Rutherford backscattering spectrometry (RBS) and measure by X-ray photoelectron spectrometry (XPS) an oleylamine surface coverage of 2.3 ligands per nm². X-ray diffraction experiments and simulations as well as high-resolution dark-field STEM point toward a P3m1 trigonal crystallographic structure (g phase). Transport measurements show that the nanoribbons display n-type semiconductor unipolar behavior. Their lateral dimensions can be tuned by reaction time, temperature and by the amount of water present in the reaction medium: anhydrous synthesis conditions lead to hexagonal nanoplates, whereas controlled addition of water induces a symmetry break yielding long rectangular nanoribbons. Depending on the dispersion solvent, these long ribbon-like nanoparticles can form either well-dispersed colloids or bundles in which they stack face-to-face. Their large aspect ratio induces the formation of gels at volume fractions as low as 1.3 × 10^-4 making them supergelators. The kinetics of gelation is strongly accelerated by an increase in the relative humidity of the ambient atmosphere.