The third ventricle is one of the four connected fluid-filled cavities within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid (CSF).

It is in the midline, between the left and right lateral ventricles. It communicates with the lateral ventricles anteriorly (in front), and with the mesencephalic duct (aqueduct of Sylvius) posteriorly (in back).

Developmentally it represents the cavity of the diencephalon, though they are in front of the interventricular foramen which is derived from the median part of the telencephalon. The third ventricle is bounded by the thalamus and hypothalamus on both the left and right sides. The lamina terminalis forms the anterior wall of the third ventricle.

There are two protrusions on the front of the third ventricle, the supra-optic recess (above the optic chiasma), and the infundibular recess (above the pituitary stalk). In casts of the ventricular system, a small 'hole' may be seen in the body of the third ventricle. This is formed where the two thalami are joined together at the interthalamic adhesion (not seen in all people).

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.