A measurement of the energy dependence of antineutrino disappearance at the Daya Bay reactor neutrino experiment is reported. Electron antineutrinos (ν̄e) from six 2.9 GWth reactors were detected with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls. Using 217 days of data, 41 589 (203 809 and 92 912) antineutrino candidates were detected in the far hall (near halls). An improved measurement of the oscillation amplitude sin22θ13=0.090-0.009+0. 008 and the first direct measurement of the ν̄e mass-squared difference |Δmee2|=(2.59-0.20+0.19)×10-3 eV2 is obtained using the observed ν̄e rates and energy spectra in a three-neutrino framework. This value of |Δmee2| is consistent with |Δmμμ2| measured by muon neutrino disappearance, supporting the three-flavor oscillation model. © 2014 American Physical Society.