  We report the discovery of KELT-20b, a hot Jupiter transiting a V~7.6 early A
star with an orbital period of P~3.47 days. We identified the initial transit
signal in KELT-North survey data. Archival and follow-up photometry, the Gaia
parallax, radial velocities, Doppler tomography, and adaptive optics imaging
were used to confirm the planetary nature of the companion and characterize the
system. From global modeling we infer that the host star HD 185603 is a
rapidly-rotating (VsinI~120 km/s) A2V star with an effective temperature of
$T_{eff}$=8730K, mass of $M_{star}=1.76M_{sun}$, radius of
$R_{star}=1.561R_{sun}$, surface gravity of logg=4.292, and age of <600 Myr.
The planetary companion has a radius of $1.735^{+0.070}_{-0.075}~R_{J}$, a
semimajor axis of $a=0.0542^{+0.0014}_{-0.0021}$AU, and a linear ephemeris of
$BJD_{TDB}=2457503.120049 \pm 0.000190 + E(3.4741070\pm0.0000019)$. We place a
$3\sigma$ upper limit of ~3.5 $M_{J}$ on the mass of the planet. The Doppler
tomographic measurement indicates that the planetary orbit is well aligned with
the projected spin-axis of the star ($\lambda= 3.4\pm {2.1}$ degrees). The
inclination of the star is constrained to be $24.4<I_*<155.6$ degrees, implying
a true (three-dimensional) spin-orbit alignment of $1.3<\psi<69.8$ degrees. The
planet receives an insolation flux of $\sim 8\times 10^9~{\rm
erg~s^{-1}~cm^{-2}}$, implying an equilibrium temperature of of ~ 2250 K,
assuming zero albedo and complete heat redistribution. Due to the high stellar
$T_{eff}$, the planet also receives an ultraviolet (wavelengths $d\le 91.2$~nm)
insolation flux of $\sim 9.1\times 10^4~{\rm erg~s^{-1}~cm^{-2}}$, which may
lead to significant ablation of the planetary atmosphere. Together with
WASP-33, Kepler-13 A, HAT-P-57, KELT-17, and KELT-9, KELT-20 is the sixth A
star host of a transiting giant planet, and the third-brightest host (in V) of
a transiting planet.
