Although there is consensus that instrumental conditioning depends on the encoding of action–outcome associations, it is not known where this learning process is localized in the brain. Recent research suggests that the posterior dorsomedial striatum (pDMS) may be the critical locus of these associations. We tested this hypothesis by examining the contribution of N-methyl-d-aspartate receptors (NMDARs) in the pDMS to action–outcome learning. Rats with bilateral cannulae in the pDMS were first trained to perform two actions (left and right lever presses), for sucrose solution. After the pre-training phase, they were given an infusion of the NMDA antagonist 2-amino-5-phosphonopentanoic acid (APV, 1 mg/mL) or artificial cerebral spinal fluid (ACSF) before a 30-min session in which pressing one lever delivered food pellets and pressing the other delivered fruit punch. Learning during this session was tested the next day by sating the animals on either the pellets or fruit punch before assessing their performance on the two levers in extinction. The ACSF group selectively reduced responding on the lever that, in training, had earned the now devalued outcome, whereas the APV group did not. Experiment 2 replicated the effect of APV during the critical training session but found no effect of APV given after acquisition and before test. Furthermore, Experiment 3 showed that the effect of APV on instrumental learning was restricted to the pDMS; infusion into the dorsolateral striatum did not prevent learning. These experiments provide the first direct evidence that, in instrumental conditioning, NMDARs in the dorsomedial striatum are involved in encoding action–outcome associations.