Motor adaptation is crucial for performing accurate movements in a changing environment and relies on the cerebellum. Although cerebellar involvement has been well characterized, the neurochemical changes in the cerebellum that underpin human motor adaptation remain unknown. We used a novel Magnetic Resonance Spectroscopic Imaging (MRSI) technique to measure changes in the major inhibitory neurotransmitter γ-aminobutyric acid (GABA) in the human cerebellum during visuomotor adaptation. Participants used their right hand to adapt to a rotated cursor in the scanner, compared with a control task requiring no adaptation. We were able to spatially resolve adaptation-driven GABA changes at the cerebellar nuclei and in the cerebellar cortex in the left and the right cerebellar hemisphere independently and found that simple movement of the right hand increases GABA in the right cerebellar nuclei and decreases GABA in the left. When isolating adaptation-driven GABA changes, we found an increase in GABA in the left cerebellar nuclei and a decrease in GABA in the right cerebellar nuclei during adaptation. Early adaptation-driven GABA change in the right cerebellar nuclei correlated with adaptation performance: Participants showing greater GABA decrease adapted better, suggesting that this early GABA change is behaviourally relevant. Early GABA change also correlated with functional connectivity change in a cerebellar network: Participants showing a greater decrease in GABA also showed greater strength increase in cerebellar network connectivity. These results were specific to GABA, specific to adaptation and specific to the cerebellar network. This study provides the first evidence for plastic changes in cerebellar neurochemistry during a motor adaptation task. Characterising these naturally occurring neurochemical changes may provide a basis for developing therapeutic interventions to facilitate neurochemical changes in the cerebellum that can improve human motor adaptation.