The lentiform nucleus (or lentiform complex, lenticular nucleus, or lenticular complex) are the putamen (laterally) and the globus pallidus (medially), collectively. Due to their proximity, these two structures were formerly considered one, however, the two are separated by a thin layer of white matter—the external medullary lamina—and are functionally and connectionally distinct.

The lentiform nucleus is a large, lens-shaped mass of gray matter just lateral to the internal capsule. It forms part of the basal ganglia. With the caudate nucleus, it forms the dorsal striatum.

When divided horizontally, it exhibits, to some extent, the appearance of a biconvex lens, while a coronal section of its central part presents a somewhat triangular outline.

It is shorter than the caudate nucleus and does not extend as far forward.

It is deep/medial to the insular cortex, with which it is coextensive; the two are separated by intervening structures.

It is lateral to the caudate nucleus and thalamus, and is seen only in sections of the hemisphere.

It is bounded laterally by a lamina of a white substance called the external capsule, and lateral to this is a thin layer of gray substance termed the claustrum.

Its anterior end is continuous with the lower part of the head of the caudate nucleus and with the anterior perforated substance.