The Daya Bay Reactor Neutrino Experiment has measured a nonzero value for the neutrino mixing angle 0(13) with a significance of 5.2 standard deviations. Antineutrinos from six 2.9 GW(th) reactors were detected in six antineutrino detectors deployed in two near (flux-weighted baseline 470 m and 576 m) and one far (1648 m) underground experimental halls. With a 43 000 ton-GW(th)-day live-time exposure in 55 days, 10 416 (80 376) electron-antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to expected number of antineutrinos at the far hall is R = 0.940 +/- 0.011(stat.) +/- 0.004(syst.). A rate-only analysis finds sin(2)2 theta(13) = 0.092 +/- 0.016(stat.) +/- 0.005(syst.) in a three-neutrino framework.