People instantaneously evaluate faces with significant agreement on evaluations of social traits. However, the neural basis for such rapid spontaneous face evaluation remains largely unknown. Here, we recorded from 490 neurons in the human amygdala and hippocampus and found that the neuronal activity was associated with the geometry of a social trait space. We further investigated the temporal evolution and modulation on the social trait representation, and we employed encoding and decoding models to reveal the critical social traits for the trait space. We also recorded from another 938 neurons and replicated our findings using different social traits. Together, our results suggest that there exists a neuronal population code for a comprehensive social trait space in the human amygdala and hippocampus that underlies spontaneous first impressions. Changes in such neuronal social trait space may have implications for the abnormal processing of social information observed in some neurological and psychiatric disorders.