NMDA (N-methyl-D-aspartic acid) is an amino acid derivative acting as specific agonist at the NMDA receptor, and therefore mimics the action of the neurotransmitter glutamate on that receptor. In contrast to glutamate it binds to and opens the above receptor only, but essentially not to other glutamate receptors.

NMDA is a water-soluble synthetic substance not normally found in biological tissue.

Examples of inhibitors of the NMDA receptor are APV, dextromethorphan, phencyclidine (PCP) and riluzole. They are commonly referred to as NMDA receptor antagonists.