We present the discovery of HAT-P-57b, a P = 2.4653 day transiting planet around a V=10.465 /- 0.029 mag, {T}{{eff}}=7500 /- 250 K main sequence A8V star with a projected rotation velocity of v{sin}i=102.1 /- 1.3 {km} {{{s}}}-1. We measure the radius of the planet to be R=1.413 /- 0.054 {R}{{J}} and, based on RV observations, place a 95% confidence upper limit on its mass of M\lt 1.85 {M}{{J}}. Based on theoretical stellar evolution models, the host star has a mass and radius of 1.47 /- 0.12 {M}ȯ and 1.500 /- 0.050 {R}ȯ , respectively. Spectroscopic observations made with Keck-I/HIRES during a partial transit event show the Doppler shadow of HAT-P-57b moving across the average spectral line profile of HAT-P-57, confirming the object as a planetary system. We use these observations, together with analytic formulae that we derive for the line profile distortions, to determine the projected angle between the spin axis of HAT-P-57 and the orbital axis of HAT-P-57b. The data permit two possible solutions, with -16\buildrel{\circ}\over{.} 7\lt λ \lt 3\buildrel{\circ}\over{.} 3 or 27\buildrel{\circ}\over{.} 6\lt λ \lt 57\buildrel{\circ}\over{.} 4 at 95% confidence, and with relative probabilities for the two modes of 26% and 74%, respectively. Adaptive optics imaging with MMT/Clio2 reveals an object located 2\buildrel{\prime\prime}\over{.} 7 from HAT-P-57 consisting of two point sources separated in turn from each other by 0\buildrel{\prime\prime}\over{.} 22. The H- and {L}\prime -band magnitudes of the companion stars are consistent with their being physically associated with HAT-P-57, in which case they are stars of mass 0.61 /- 0.10 {M}ȯ and 0.53 /- 0.08 {M}ȯ . HAT-P-57 is the most rapidly rotating star, and only the fourth main sequence A star, known to host a transiting planet. Based on observations obtained with the Hungarian-made Automated Telescope Network. Based in part on observations made with the Keck-I telescope at Mauna Kea Observatory, HI (Keck time awarded through NASA programs N029Hr, N108Hr, N154Hr and N130Hr and NOAO programs A289Hr, and A284Hr). Based in part on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. Based in part on observations obtained with the Tillinghast Reflector 1.5 m telescope and the 1.2 m telescope, both operated by the Smithsonian Astrophysical Observatory at the Fred Lawrence Whipple Observatory in Arizona.
