Human motor skill acquisition is improved by performance feedback, and coupling such feedback with extrinsic reward (such as money) can enhance skill learning. However, the neurophysiology underlying such behavioral effect is unclear. To bridge this gap, we assessed the effects of reward on multiple forms of motor plasticity during skill learning. Sixty-five healthy participants divided into three groups performed a pinch-grip skill task with sensory feedback only, sensory and reinforcement feedback, or both feedback coupled with an extrinsic monetary reward during skill training. To probe motor plasticity, we applied transcranial magnetic stimulation at rest, on the left primary motor cortex before, at an early-training time point, and after training in the three groups and measured motor-evoked potentials from task-relevant muscle of the right arm. This allowed us to evaluate the amplitude and variability of corticospinal output, GABAergic short-intracortical inhibition, and use-dependent plasticity before training and at two additional time points (early and end training). At the behavioral level, monetary reward accelerated skill learning. In parallel, corticospinal output became less variable early on during training in the presence of extrinsic reward. Interestingly, this effect was particularly pronounced for participants who were more sensitive to reward, as evaluated in an independent questionnaire. Other measures of motor excitability remained comparable across groups. These findings highlight that a mechanism underlying the benefit of reward on motor skill learning is the fine-tuning of early-training resting-state corticospinal variability.