The claustrum is a small subcortical nucleus present in all placental mammals, that shares extensive and reciprocal connections with all areas of the cortex. Little is known about its structure, circuit organization and function. By performing retrograde and anterograde tracing, we demonstrated that claustrum projection neurons are topographically segregated in projection subfields along the dorsoventral axis of the claustrum depending on their cortical target. By patch clamp recording, we determined that the neuronal composition of the different subfields is consistent along the extension of the claustrum. We then dissected the internal circuit of the claustrum by optogenetics. Although different projection subfields are reciprocally connected with different cortical areas, they are strongly interconnected with each other within the local circuit of the claustrum. These results contribute to increase our understanding of claustrum internal functioning, and open new possibilities in determining the role of this nucleus in whole brain computation.