The MiniBooNE experiment at Fermilab reports results from a search for ν̅ μ→ν̅ e oscillations, using a data sample corresponding to 5.66×1020 protons on target. An excess of 20.9±14.0 events is observed in the energy range 475<EνQE<1250  MeV, which, when constrained by the observed ν̅ μ events, has a probability for consistency with the background-only hypothesis of 0.5%. On the other hand, fitting for ν̅ μ→ν̅ e oscillations, the best-fit point has a χ2 probability of 8.7%. The data are consistent with ν̅ μ→ν̅ e oscillations in the 0.1 to 1.0  eV2 Δm2 range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.