It is in the midline, between the left and right lateral ventricles. It communicates with the lateral ventricles anteriorly (in front), and with the 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 either side.