We report the discovery of KELT-10b, the first transiting exoplanet discovered using the KELT-South telescope. KELT-10b is a highly inflated sub-Jupiter mass planet transiting a relatively bright V = 10.7 star (TYC 8378-64-1), with T_(eff) = 5948 ± 74 K, log g = 4.319^(+0.020)_(−0.030) and [Fe/H] = 0.09^(+0.11)_(−0.10), an inferred mass M^* = 1.112^(+0.055)_(−0.061) M_⊙ and radius R^* = 1.209^(+0.047)_(−0.035) R_⊙. The planet has a radius Rp = 1.399^(+0.069)_(−0.049) RJ and mass Mp = 0.679^(+0.039)_(−0.038) MJ. The planet has an eccentricity consistent with zero and a semimajor axis a = 0.05250^(+0.00086)_(−0.00097) au. The best-fitting linear ephemeris is T_0 = 2457 066.720 45 ± 0.000 27 BJD_(TDB) and P = 4.166 2739 ± 0.000 0063 d. This planet joins a group of highly inflated transiting exoplanets with a larger radius and smaller mass than that of Jupiter. The planet, which boasts deep transits of 1.4 per cent, has a relatively high equilibrium temperature of T_(eq) = 1377^(+28)_(−23) K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817^(+0.068)_(−0.054) × 10^9 erg s^(−1) cm^(−2), which places it far above the insolation threshold above which hot Jupiters exhibit increasing amounts of radius inflation. Evolutionary analysis of the host star suggests that KELT-10b may not survive beyond the current subgiant phase, depending on the rate of in-spiral of the planet over the next few Gyr. The planet transits a relatively bright star and exhibits the third largest transit depth of all transiting exoplanets with V < 11 in the Southern hemisphere, making it a promising candidate for future atmospheric characterization studies
