Descending command neurons instruct spinal networks to execute basic locomotor functions, such as gait and speed. The command functions for gait and speed are symmetric, implying that a separate unknown system directs asymmetric movements, including the ability to move left or right. In the present study, we report that Chx10-lineage reticulospinal neurons act to control the direction of locomotor movements in mammals. Chx10 neurons exhibit mainly ipsilateral projection, and their selective unilateral activation causes ipsilateral turning movements in freely moving mice. Unilateral inhibition of Chx10 neurons causes contralateral turning movements. Paired left-right motor recordings identified distinct mechanisms for directional movements mediated via limb and axial spinal circuits. Finally, we identify sensorimotor brain regions that project on to Chx10 reticulospinal neurons, and demonstrate that their unilateral activation can impart left-right directional commands. Together these data identify the descending motor system that commands left-right locomotor asymmetries in mammals.