Neutron scattering demonstrates the coexistence of antiferromagnetic order and superconductivity below 1 K in the heavy-electron system URu2Si2. It is found that the upper (17.5 K) transition is to an antiferromagnetic phase with a (100) modulation wave vector and spins along the tetragonal c axis. The ordered moment is unusually small, (0.03 ± 0.01)μB. However, spin waves develop from damped, finite-gap excitations above TN, and they are intense, propagating, and longitudinal with a zone-center gap of 1.8 meV