The third ventricle is one of the four ventricles in the brain that communicate with one another. As with the other ventricles of the brain, it is filled with cerebrospinal fluid, which helps to protect the brain from injury and transport nutrients and waste. The third ventricle is a narrow cavity that is located between the two halves of the brain.

The third ventricle sends messages to and receives messages from the lateral ventricles, which are located in front of the third ventricle, and the aqueduct of the midbrain, which is located directly behind the third ventricle. The hypothalamus and thalamus are located on the sides of the third ventricle.

Abnormalities of the third ventricle are associated with various conditions including hydrocephalus, meningitis, and ventriculitis. Hydrocephalus is an excessive buildup of fluid on the brain. Meningitis is inflammation of the membranes that cover the brain and spinal cord, whereas ventriculitis is an inflammatory condition of the ventricles. Meningitis and ventriculitis can both be caused by trauma to a ventricle, including the third ventricle, althought traumatic meningitis is rare.

An enlarged third ventricle has been associated with psychiatric illnesses such as schizophrenia; however, the accuracy of this theory hasn't been conclusively proven.