Learning from rewards and punishments is crucial for survival, and is mediated by interactions between the prefrontal-cortex (PFC) and subcortical structures. It is thought that inhibition plays a key role in these interactions, yet there is little direct evidence for this in humans. Here we overcome the technical challenges of measuring inhibition in the human brain and show that the primary inhibitory neurotransmitter gamma-aminobutyric-acid (GABA) differentially affects the interaction between the dorsal-anterior-cingulate-cortex (dACC) and subcortical structures during learning with rewards versus punishments. Using a multimodal magnetic-resonance-spectroscopy (MRS) and whole-brain BOLD (fMRI) activity approach at ultrahigh fields (7T) in a cohort of 111 healthy volunteers, we found that higher levels of GABA in the dorsal anterior cingulate cortex (dACC) were associated with decreased learning performance and reduced activity in the putamen during reward learning. Conversely, during punishment learning, GABA levels in the dACC were negatively correlated with the functional connectivity between the dACC and the putamen, suggesting a role in suppressing less relevant signals. Our results show that inhibition in the dACC mediates appetitive and aversive learning in humans through distinct mechanisms and emphasize the contribution of spectroscopic data to elucidating the role of inhibitory mechanisms in human valence-based behavior.