Coordinated triadic interactions, involving oneself, another person, and an external object, are considered a uniquely human skill. However, the exact mechanisms underlying the ability to engage in such social interactions remain hitherto unknown. We used functional neuroimaging to investigate the neural signature of triadic interactions. For this purpose, participants viewed pictures of objects in a 3 Tesla functional magnetic resonance imaging scanner and were asked whether they could imagine this object in a social interaction with another person. We also aimed to dissociate this process from, as well as to find commonalities with, purely self-referential or other-referential processing. In all trial-types, we found activations in core mentalizing brain areas (medial prefrontal cortex, posterior cingulate cortex, precuneus and temporoparietal junction). Furthermore, triadic engagements, but not self-referential or other-referential processing, were associated with activations in classical mirror neuron areas (inferior frontal gyrus and inferior parietal lobe). Finally, mentalizing networks showed a strong functional connectivity with mirror neuron areas exclusively during triadic engagements. These results suggest that the imagined interaction of two agents is processed in a more complex neural social cognitive network than purely self- or other-referential considerations.