This study used event-related potential (ERP) and retrieval practice effect paradigm to investigate the neurocognitive mechanisms underlying the retrieval practice effect in a third language (L3) vocabulary learning. Thirty-five Chinese (First Language, L1)-English (Second Language, L2) bilinguals without prior knowledge of French (L3) studied 120 French (L3)-English (L2) word pairs and then practiced the word pairs through retrieval practice or repeated studying. On a final recognition test 30 minutes later, the participants were asked to perform old/new recognition judgments while recording electroencephalography (EEG) signals. Behavioural results found that retrieval practice led to a higher accuracy rate and faster response time in the final test than repeated studying. ERP results found that retrieval practice elicited less negative N400 waveforms than repeated studying at the frontal site and less negative N400 waveforms than new condition at the central-parietal sites. By contrast, retrieval practice only elicited more positive LPC waveforms than new condition at the parietal site. In sum, the present results provide first neural evidence that retrieval practice facilitates L3 (French) vocabulary learning. These findings suggest that retrieval practice might involve more semantic information processing in L3 vocabulary learning, which supports the elaborative retrieval hypothesis of the retrieval practice effect.