HATS-8b is a low density transiting super-Neptune discovered as part of the HATSouth project. The planet orbits its solar-like G-dwarf host (V=14.03+/-0.10, T_eff_=50K) with a period of 3.5839days. HATS-8b is the third lowest-mass transiting exoplanet to be discovered from a wide-field ground-based search, and with a mass of 0.138+/0.019M_J_ it is approximately halfway between the masses of Neptune and Saturn. However, HATS-8b has a radius of 0.873^+0.123^-0.075, resulting in a bulk density of just 0.259+/-0.091g/cm3. The metallicity of the host star is super-solar ([Fe/H]=0.210+/-0.080), providing evidence against the idea that low-density exoplanets form from metal-poor environments. The low density and large radius of HATS-8b results in an atmospheric scale height of almost 1000km, and in addition to this there is an excellent reference star of nearly equal magnitude at just 19'' separation in the sky. These factors make HATS-8b an exciting target for future atmospheric characterization studies, particularly for long-slit transmission spectroscopy.