Objective Internal globus pallidus (GPi) deep brain stimulation (DBS) relieves symptoms in dystonia patients. However, the physiological effects produced by GPi DBS are not fully understood. In particular, how a single‐pulse GPi DBS changes cortical circuits has never been investigated. We studied the modulation of motor cortical excitability and plasticity with single‐pulse GPi DBS in dystonia patients with bilateral implantation of GPi DBS. Methods The cortical evoked potentials from DBS were recorded with electroencephalography. Transcranial magnetic stimulation with a conditioning test paired‐pulse paradigm was used to investigate the effect of GPi DBS on the primary motor cortex. How GPi DBS might modulate the motor cortical plasticity was tested using a paired associative stimulation paradigm with repetitive pairs of GPi DBS and motor cortical stimulation at specific time intervals. Results GPi stimulation produced 2 peaks of cortical evoked potentials with latencies of ∼10 and ∼25 milliseconds in the motor cortical area. Cortical facilitation was observed at ∼10 milliseconds after single‐pulse GPi DBS, and cortical inhibition was observed after a ∼25‐millisecond interval. Repetitive pairs of GPi stimulation with cortical stimulation at these 2 time intervals produced long‐term potentiation‐like effects in the motor cortex. Interpretation Single‐pulse DBS modulates cortical excitability and plasticity at specific time intervals. These effects may be related to the mechanism of action of DBS. Combination of DBS with cortical stimulation with appropriate timing has therapeutic potential and could be explored in the future as a method to enhance the effects of neuromodulation for neurological and psychiatric diseases. Ann Neurol 2018;83:352–362