Neuroimaging evidence suggests that human Brodmann area 5 (BA5) within the superior parietal lobule contributes to movement planning. However, a causal role for the contribution of BA5 to preparatory processes has yet to be reported. We used paired-pulse transcranial magnetic stimulation to investigate the influence of human BA5 on corticospinal excitability during movement preparation in the context of a GO/NO-GO task. Functional connectivity between BA5 and the ipsilateral primary motor cortex (M1) was investigated by probing corticospinal output to the first dorsal interosseous muscle of the right hand. Results indicate that BA5 influences M1 during movement preparation in a task-specific manner: motor-evoked potentials are suppressed in the context of a NO-GO versus GO task. These findings provide evidence that human BA5 participates in movement preparation and differentiates between whether a movement is withheld or executed.