Although much learning occurs through direct experience of errors, humans and other animals can learn from the errors of other individuals. The medial frontal cortex (MFC) processes self-generated errors, but the neuronal architecture and mechanisms underlying the monitoring of others' errors are poorly understood. Exploring such mechanisms is important, as they underlie observational learning and allow adaptive behavior in uncertain social environments. Using two paired monkeys that monitored each other's action for their own action selection, we identified a group of neurons in the MFC that exhibited a substantial activity increase that was associated with another's errors. Nearly half of these neurons showed activity changes consistent with general reward-omission signals, whereas the remaining neurons specifically responded to another's erroneous actions. These findings indicate that the MFC contains a dedicated circuit for monitoring others' mistakes during social interactions.