We report on the discovery and validation of Kepler-452b, a transiting planet identified by a search through the 4 years of data collected by NASA's Kepler Mission. This possibly rocky 1.63_(-0.20)^(+0.23) R⨁ planet orbits its G2 host star every 384.843_(-0.012)^(+0.007) days, the longest orbital period for a small (R_p < 2 R⨁) transiting exoplanet to date. The likelihood that this planet has a rocky composition lies between 49% and 62%. The star has an effective temperature of 5757 ± 85 K and a log g of 4.32 ± 0.09. At a mean orbital separation of 1.046_(-0.015)^(+0.019) AU, this small planet is well within the optimistic habitable zone of its star (recent Venus/early Mars), experiencing only 10% more flux than Earth receives from the Sun today, and slightly outside the conservative habitable zone (runaway greenhouse/maximum greenhouse). The star is slightly larger and older than the Sun, with a present radius of 1.11_(-0.09)^(+0.15) R⨁ and an estimated age of ~6 Gyr. Thus, Kepler-452b has likely always been in the habitable zone and should remain there for another ~3 Gyr
