We report the discovery by the HATSouth survey of HATS-6b , an extrasolar planet transiting a V= 15.2 mag , i = 13.7 mag M1V star with a mass of 0.57 M _ { \sun } and a radius of 0.57 R _ { \sun } . HATS-6b has a period of P \approx 3.3253 d , mass of M _ { p } \approx 0.32 M _ { J } , radius of R _ { p } \approx 1.00 R _ { J } , and zero-albedo equilibrium temperature of T _ { eq } = 712.8 \pm 5.1 K. HATS-6 is one of the lowest mass stars known to host a close-in gas giant planet , and its transits are among the deepest of any known transiting planet system . We discuss the follow-up opportunities afforded by this system , noting that despite the faintness of the host star , it is expected to have the highest K -band S/N transmission spectrum among known gas giant planets with T _ { eq } < 750 K. In order to characterize the star we present a new set of empirical relations between the density , radius , mass , bolometric magnitude , and V , J , H and K -band bolometric corrections for main sequence stars with M < 0.80 M _ { \sun } , or spectral types later than K5 . These relations are calibrated using eclipsing binary components as well as members of resolved binary systems . We account for intrinsic scatter in the relations in a self-consistent manner . We show that from the transit-based stellar density alone it is possible to measure the mass and radius of a \sim 0.6 M _ { \sun } star to \sim 7 % and \sim 2 % precision , respectively . Incorporating additional information , such as the V - K color , or an absolute magnitude , allows the precision to be improved by up to a factor of two .