The fornix commissure is a thin, triangular sheet of transverse (horizontal) fibers. It is located on the inside of the brain's fornix, which is the main structure of the hippocampal formation, situated at the midline of the brain. It is commonly known as either the commissure of fornix or hippocampal commissure.

The fornix of brain is a cluster of axons (part of a nerve cell) that is responsible for transmitting signals between the mammillary bodies, the septal nuclei, and the hippocampus — parts of the brain that play a role in memory storage and recall.

The fornix is C-shaped with two projections in the front and two in the back, called pillars. The commissure attaches to the fornix at the medial (middle) edges of the posterior (back) pillars. The fornix commissure is situated near the uncus (part of the temporal lobe), the anterior (front) pillars of fornix, the fimbria (fringe around the hippocampus), and the crus fornicis (the posterior pillars of the fornix).

The back sections of the fornix are connected together by the psalterium, which is a thin lamina (a type of tissue). The psalterium, which is also known by the name lyra, consists of transverse fibers, which bring together the two hippocampi that lie at the end of the posterior pillars of the fornix. The end of the lamina forms the plate of the commissure. This plate connects to the septum pellucidum, the corpus callosum, and the fornix, structures that lie in the midline of the brain.