Lesion or degeneration of the cerebellum can profoundly impair adaptive control of reaching in humans. Computational models have proposed that internal models for motor action form in the cerebellum and influence planned motor output through the cerebello-thalamo-cortical pathway. However, lesions in the cerebellar thalamus have not been consistently shown to result in motor adaptation impairment. To elucidate the role of the cerebellar thalamus in humans, we studied a group of essential tremor (ET) patients with deep brain stimulation (DBS) electrodes placed in the cerebellar thalamus. The stimulation can be turned on or off remotely, and is thought to reduce tremor by blocking the spread of the pathological output from the cerebellum. We found that while thalamic DBS resulted in a dramatic reduction in ET, it also led to impaired motor adaptation in the patients. Thus, human adaptive reaching motor control depends on the integrity of the thalamic nucleus that relays cerebellar output to the cerebral cortex.