Learning the causal structures of social environments involves predicting significant events (e.g., rewards) and detecting prediction errors for each agent. Whether the brain can simultaneously compute reward prediction errors for self (S-RPE) and others (O-RPE), and which neurons are responsible, is unclear. Here, we condition two monkeys with identical visual stimuli predicting different reward outcomes and find that dorsomedial prefrontal neurons represent both S-RPE and O-RPE simultaneously. Neuronal signatures of RPE are agent and sign specific, forming distinct populations for positive and negative S-RPE and O-RPE. A linear decoder trained on neurons encoding O-RPE, but not S-RPE, successfully discriminates RPE. Further investigation identifies coexisting actual reward and prediction confirmation signals for others. These results highlight the presence of neuronal mechanisms in the primate brain that update the value of environmental stimuli simultaneously for oneself and others, enabling primates to comprehend the causal structure of the world from the perspective of others.