We report the discovery of KELT-20b, a hot Jupiter transiting a V ∼7.6 early A star, HD 185603, with an orbital period of P ≃3.47 days. Archival and follow-up photometry, Gaia parallax, radial velocities, Doppler tomography, and AO imaging were used to confirm the planetary nature of KELT-20b and characterize the system. From global modeling we infer that KELT-20 is a rapidly rotating (v sin I∗ ≃ 120 km s-1 ) A2V star with an effective temperature of Teff=8730 -260+250 K, mass of M∗+1.76-0.20+0.14MO, radius of R∗= 1.561-0.064+0.058RO,surface gravity of log g ∗=4.292-0.020+0.017, and age of ≳600 Myr. The planetary companion has a radius of Rp=1.735 -0.075+0.070Rj, a semimajor axis of a =0.0542-0.0021+0.0014 au, and a linear ephemeris of BJDTDB=2457503.120049 ± 0.000190+E(3.4741070± 0.0000019). We place a 3s upper limit of ∼ 3.5 MJ on the mass of the planet. Doppler tomographic measurements indicate that the planetary orbit normal is well aligned with the projected spin axis of the star (l = 3 °.4± 2 °.1). The inclination of the star is constrained to 24°.4 > I ∗ > 155 °.6, implying a three-dimensional spin-orbit alignment of 1°.3 > ψ > 69°.8. KELT-20b receives an insolation flux of∼8× 109 erg s-1cm-2, implying an equilibrium temperature of of ∼ 2250 K, assuming zero albedo and complete heat redistribution. Due to the high stellar Teff, KELT-20b also receives an ultraviolet (wavelength d ≤ 91.2 nm) insolation flux of ∼9.1×104erg s-1 cm-2, possibly indicating significant atmospheric ablation. 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 thirdbrightest host (in V) of a transiting planet.
