We report on the discovery of a new extremely short period transiting extrasolar planet, WASP-19b. The planet has mass Mpl = 1.15 ± 0.08 MJ, radius Rpl = 1.31 ± 0.06 RJ, and orbital period P=0.7888399 ± 0.0000008 days. Through spectroscopic analysis, we determine the host star to be a slightly super-solar metallicity ([M/H]=0.1 ± 0.1 dex) G-dwarf with Teff=5500 ± 100 K. In addition, we detect periodic, sinusoidal flux variations in the light curve which are used to derive a rotation period for the star of Prot = 10.5 ±0.2 days. The relatively short stellar rotation period suggests that either WASP-19 is somewhat young (~ 600 Myr old) or tidal interactions between the two bodies have caused the planet to spiral inward over its lifetime resulting in the spin-up of the star. Due to the detection of the rotation period, this system has the potential to place strong constraints on the stellar tidal quality factor, Q's, if a more precise age is determined.
