The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles. The shape of the gland resembles a pine cone, which gives it its name. The pineal gland is located in the epithalamus, near the center of the brain, between the two hemispheres, tucked in a groove where the two halves of the thalamus join. It is one of the neuroendocrine secretory circumventricular organs in which capillaries are mostly permeable to solutes in the blood.

The pineal gland is present in almost all vertebrates, but is absent in protochordates in which there is a simple pineal homologue. The hagfish, considered as a primitive vertebrate, has a rudimentary structure regarded as the "pineal equivalent" in the dorsal diencephalon. In some species of amphibians and reptiles, the gland is linked to a light-sensing organ, variously called the parietal eye, the pineal eye or the third eye. Reconstruction of the biological evolution pattern suggests that the pineal gland was originally a kind of atrophied photoreceptor that developed into a neuroendocrine organ.

Ancient Greeks were the first to notice the pineal gland and believed it to be a valve, a guardian for the flow of pneuma. Galen in the 2nd century C.E. could not find any functional role and regarded the gland as a structural support for the brain tissue. He gave the name konario, meaning cone or pinecone, which during Renaissance was translated to Latin as pinealis. In the 17th century, René Descartes revived the mystical purpose and described the gland as the "principal seat of the soul". In the mid-20th century, the real biological role as a neuroendocrine organ was established.