Low-frequency oscillations in motor cortex local field potentials (LFP) are suppressed around movement onset. This suppression starts already during the preparatory period. Previous studies suggest that cortico-subcortical networks contribute to this suppression. The motor thalamus, which is a part of this loop, is strongly modulated by cerebellar inputs. We investigated the impact of cerebellar signals on motor cortical beta activity in monkeys trained to perform a delayed reaching task. The cerebellar outflow was reversibly blocked using high-frequency stimulation (130 Hz) in the superior cerebellar peduncle. This manipulation resulted in reduced beta suppression in cortical LFPs during the delay period and increased the reaction times of subsequent movements, reflecting impaired movement preparation. Cerebellar block also reduced the correlation between preparatory beta power and reaction times, indicating an increased variability in movement initiation. These findings highlight the role of the cerebellum in coordinating motor cortical activity thereby ensuring effective movement preparation.