The Λb polarization in hadronic Z decays is measured in semileptonic decays from the average energies of the charged lepton and the neutrino. In a data sample of approximately 3 million hadronic Z decays collected by the ALEPH detector at LEP between 1991 and 1994, 462 ± 31 Λb candidates are selected using (Λπ+)-lepton correlations. From this event sample, the Λb polarization is measured to be Λb = −0.23−0.20+0.24(stat.)−0.07+0.08(syst.).