  We report an observation of new bottom baryons produced in proton-antiproton
collisions at the Tevatron. Using 1.1 fb^{-1} of data collected by the CDF II
detector, we observe four \Lambda_b^0\pi^{\pm} resonances in the fully
reconstructed decay mode \Lambda_b^0 \to \Lambda_c^+ \pi^-, where \Lambda_c^+
\to p K^- \pi^+. We interpret these states as the \Sigma_b^{(*)\pm} baryons and
measure the following masses: m_{\Sigma_b^+} = 5807.8^{+2.0}_{-2.2}(stat.) \pm
1.7(syst.) MeV/c^2, m_{\Sigma_b^-} = 5815.2 \pm 1.0(stat.) \pm 1.7(syst.)
MeV/c^2, and m(\Sigma_b^*) - m(\Sigma_b) = 21.2^{+2.0}_{-1.9}(stat.)
^{+0.4}_{-0.3}(syst.) MeV/c^2.
