The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. It extends from the cerebral aqueduct (aqueduct of Sylvius) to the obex , and is filled with cerebrospinal fluid (CSF).

It has a characteristic diamond shape in cross-sections of the human brain. If you see the fourth ventricle in cross-sections, then you are in the pons or in the upper part of the medulla.

CSF entering the fourth ventricle through the cerebral aqueduct can exit to the subarachnoid space of the spinal cord through two lateral foramina of Luschka (singular: foramen of Luschka) and a single, midline foramen of Magendie. (See List of human anatomical parts named after people.)

The fourth ventricle has a "roof" dorsally and a "floor" ventrally.

The "roof" of the fourth ventricle is formed by the cerebellum. The "walls" are formed by the cerebellar peduncles .

Among the prominent features of the floor of the fourth ventricle are the:

• facial colliculus (formed by the internal part of the facial nerve as it loops around the abducens nucleus in the lower pons);

• sulci limitans (singular: sulcus limitans) which represent the border between the alar plate and the basal plate of the developing neural tube.

• obex, representing the caudal tip of the fourth ventricle. The obex is also a marker for the level of the foramen magnum of the skull and therefore is a marker for the imaginary dividing line between the medulla and spinal cord.

The four fluid-filled cavities in the brain, collectively the ventricular system, are the left and right lateral ventricles, the third ventricle, and the fourth ventricle.