To investigate whether a period of continuous theta burst stimulation (cTBS) over the supplementary motor area (SMA) induces cortical plasticity and thus improves bradykinesia in Parkinson's disease (PD) in the medication ON and OFF state. In total, 26 patients with Parkinson's disease were tested with both real and sham stimulation. The group was divided into an OFF-medication (4 females, mean age 65 years, disease duration 6 years) and an ON-medication group (7 females, mean age 61 years, disease duration 7 years) with each containing 13 individuals. Both groups were evaluated in terms of electrophysiological (motor-evoked potentials) and behavioural [Purdue Pegboard test (PPT), UPDRS motor subscore] parameters before (baseline condition) and after a 40-second period of real or sham continuous theta burst stimulation over the SMA ON and OFF dopaminergic drugs. Patients in the OFF group demonstrated an improved UPDRS III score (p < 0.05) and a better performance in the PPT for the less affected side (p < 0.025) compared to baseline after real stimulation. However, electrophysiological parameters did not change in either the ON or the OFF state. cTBS over the SMA has a mild effect on motor symptoms of the upper limb in the OFF state of PD patients. In contrast, stimulation did not change cortico-spinal excitability. A lack of change (i.e. no plasticity) to brain stimulation protocols is a known finding in PD. A clinical improvement in the OFF state, however, contrasts with this and the mechanism of these induced changes is worth further exploration.