We have measured the cross section and single-spin asymmetries from forward W ± → μ ± ν production in longitudinally polarized p + p collisions at √ s = 510     GeV using the PHENIX detector at the Relativistic Heavy Ion Collider. The cross sections are consistent with previous measurements at this collision energy, while the most forward and backward longitudinal single spin asymmetries provide new insights into the sea quark helicities in the proton. The charge of the W bosons provides a natural flavor separation of the participating partons.