Using polarization-resolved photoluminescence spectroscopy, we investigate the breaking of valley degeneracy by an out-of-plane magnetic field in back-gated monolayer MoSe2 devices. We observe a linear splitting of -0.22 meV/T between luminescence peak energies in σ+ and σ- emission for both neutral and charged excitons. The optical selection rules of monolayer MoSe2 couple the photon handedness to the exciton valley degree of freedom; so this splitting demonstrates valley degeneracy breaking. In addition, we find that the luminescence handedness can be controlled with a magnetic field to a degree that depends on the back-gate voltage. An applied magnetic field, therefore, provides effective strategies for control over the valley degree of freedom.