We report the discovery of WASP-6b, an inflated sub-Jupiter mass planet transiting every $ 3.3610060^{\rm + 0.0000022 }_{- 0.0000035 } $ days a mildly metal-poor solar-type star of magnitude $V = 11.9$. A combined analysis of the WASP photometry, high-precision followup transit photometry and radial velocities yield a planetary mass $M_{\rm p} = 0.503^{+0.019}_{-0.038}$MJ and radius $R_{\rm p} = 1.224^{+0.051}_{-0.052}$RJ, resulting in a density $\rho_{\rm p} = 0.27 \pm 0.05$$\rho_{\rm J}$. The mass and radius for the host star are $M_\ast = 0.88^{+0.05}_{-0.08}$$M_odot$ and $R_\ast = 0.870^{+0.025}_{-0.036}$$R_odot$. The non-zero orbital eccentricity $e = 0.054^{\rm +0.018}_{-0.015}$ that we measure suggests that the planet underwent a massive tidal heating ~1 Gyr ago that could have contributed to its inflated radius. High-precision radial velocities obtained during a transit allow us to measure a sky-projected angle between the stellar spin and orbital axis $\beta = 11^{+14}_{-18}$ deg. In addition to similar published measurements, this result favors a dominant migration mechanism based on tidal interactions with a protoplanetary disk.
