The cerebellum plays a critical role in motor learning and adaptation, which allow the motor system to acquire new skills and recalibrate following consistent perturbations. Growing evidence indicates that the cerebellum is also involved in cognition, including perception. However, so far no attempt has been made to test whether the cerebellum is similarly engaged in perceptual learning and adaptation. The present work fills this major gap by assessing perceptual learning in cerebellar patients. While maintaining central fixation, participants (4 patients with focal damage, 2 with degenerative disease, 6 controls) were asked to extrapolate the decelerating motion trajectory of a visual target and decide where it stopped within a structured visual display. Notably, the target was visible only briefly, after which it disappeared and its decelerating trajectory was to be extrapolated. Initial performance was calibrated for all observers to be at chance level. Testing continued for 8 consecutive days and improvement in performance was measured as a function of training. Compared to controls, cerebellar patients showed reduced learning, in terms of both rate and amount of improvement. A striking correlation between the extent of learning and the site of the lesion was found in focal patients, with strong deficits in patients with posterior (but not anterior) damage. These results suggest that the human cerebellum acts as a learning device for both motor and perceptual functions.