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 Teff = 5948 ± 74 K, log g = 4.319_{-0.030}^{+0.020} and [Fe/H] = 0.09_{-0.10}^{+0.11}, an inferred mass M★ = 1.112_{-0.061}^{+0.055} M☉ and radius R★ = 1.209_{-0.035}^{+0.047} R☉. The planet has a radius Rp = 1.399_{-0.049}^{+0.069} RJ and mass Mp = 0.679_{-0.038}^{+0.039} MJ. The planet has an eccentricity consistent with zero and a semi-major axis a = 0.05250_{-0.00097}^{+0.00086} AU. The best fitting linear ephemeris is T0 = 2457066.72045±0.00027 BJDTDB and P = 4.1662739±0.0000063 days. This planet joins a group of highly inflated transiting exoplanets with a radius larger and a mass less than that of Jupiter. The planet, which boasts deep transits of 1.4%, has a relatively high equilibrium temperature of Teq = 1377_{-23}^{+28} K, assuming zero albedo and perfect heat redistribution. KELT-10b receives an estimated insolation of 0.817_{-0.054}^{+0.068} × 109 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
