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Cortical beta oscillations are central to motor control. These rhythms are modulated during movement preparation and execution and are also sensitive to cognitive factors such as uncertainty and attention. However, their distinct motor and cognitive roles-roles that may inform condition-specific therapeutic strategies-remain unclear. This study aimed to characterise the respective contributions of motor and cognitive functions, and to assess their modulation with repetitive transcranial magnetic stimulation (rTMS). Twenty-four healthy participants performed a visually cued reaching task with dynamically manipulated uncertainty. Reaction time (RT) was quantified as a behavioural measure. Three stimulation conditions were applied during the preparatory periods between the 'uncertainty' and 'go' cues: no stimulation, regular rTMS at each participant's individual beta frequency, and irregular rTMS. Electroencephalography was used to measure beta-band oscillatory activity. Our results showed that 'uncertainty' cues induced bilateral beta suppression, with greater uncertainty linked to smaller reductions in beta power. Movement-related beta modulation was primarily lateralised to the hemisphere contralateral to the executing hand, where elevated pre-'go' cue beta power associated with longer RTs. Both regular and irregular rTMS significantly shortened reaction times and attenuated beta event-related desynchronisation, but regular rTMS timed to the beta down state had a significantly stronger effect. Reductions in beta desynchronisation correlated with improvements in RTs, suggesting beta desynchronisation reflects a neural threshold for movement initiation. These findings indicate that cortical beta oscillations encode distinct motor and cognitive processes, and that beta frequency rTMS can modulate beta dynamics to facilitate faster movement initiation by lowering this neural threshold. Significance Statement Beta-frequency brain rhythms are crucial for movement control but are also influenced by cognitive factors such as uncertainty. How these distinct functions are represented within beta activity has remained unclear. Using a reaching task with controlled uncertainty and non-invasive brain stimulation, this study shows that cortical beta oscillations encode separable cognitive and motor processes across brain hemispheres. Both regular and irregular rTMS shortened reaction times and attenuated beta event-related desynchronisation, with stimulation timed to the beta down state producing a stronger effect. These findings suggest that beta desynchronisation reflects a neural threshold for initiating movement and demonstrate that precisely timed brain stimulation can lower this threshold or facilitate its crossing, highlighting a potential mechanism for improving motor performance in clinical disorders.