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We report on the discovery of HAT-P-12b, a transiting extrasolar planetorbiting the moderately bright V=12.8 K4 dwarf GSC 03033-00706, with a period P= 3.2130598 +- 0.21 d, transit epoch Tc = 2454419.19556 +- 0.00020 (BJD)and transit duration 0.0974 +- 0.0006 d. The host star has a mass of 0.73 +-0.02 Msun, radius of 0.70 +- ^0.02_0.01 Rsun, effective temperature 4650 +- 60K and metallicity [Fe/H] = -0.29 +- 0.05. We find a slight correlation betweenthe observed spectral line bisector spans and the radial velocity, so weconsider, and rule out, various blend configurations including a blend with abackground eclipsing binary, and hierarchical triple systems where theeclipsing body is a star or a planet. We conclude that a model consisting of asingle star with a transiting planet best fits the observations, and show thata likely explanation for the apparent correlation is contamination fromscattered moonlight. Based on this model, the planetary companion has a mass of0.211 +- 0.012 MJup, and a radius of 0.959 +- ^0.029_0.021 RJup yielding a meandensity of 0.295 +- 0.025 g cm^-3. Comparing these observations with recenttheoretical models we find that HAT-P-12b is consistent with a ~ 1-4.5 Gyr,mildly irradiated, H/He dominated planet with a core mass Mc <~ 10 Mearth.HAT-P-12b is thus the least massive H/He dominated gas giant planet found todate. This record was previously held by Saturn.

HAT-P-12b: A Low-Density Sub-Saturn Mass Planet Transiting a Metal-Poor K Dwarf