The claustrum is a small subcortical nucleus present in reptiles, birds and mammals that has extensive reciprocal connections with the whole cortex and with subcortical areas. Thus far the claustrum has been speculated to be involved in a variety of brain mechanisms, but its precise function remains unknown. We used in vivo electrophysiological recording during freely moving behavior and fear conditioning in combination with chemogenetic tools to investigate the role of the claustrum in cortico-cortical communication. We showed that inhibition of the claustrum induced a modification in the flow of information, as well as in the co-occurrence of brain oscillations associated with memory events, across distant cortical areas. These changes were associated with alterations in the consolidation and the retrieval of fear memory. Our findings indicate that the claustrum acts as a conductor of cortico-cortical communication, and that this communication is necessary for the correct expression of fear memory.