The philtrum (Latin: philtrum, from Ancient Greek φίλτρον phíltron, lit. 'love charm') or medial cleft is a vertical indentation in the middle area of the upper lip, common to therian mammals, extending in humans from the nasal septum to the tubercle of the upper lip. Together with a glandular rhinarium and slit-like nostrils, it is believed to constitute the primitive condition for at least therian mammals. Monotremes lack a philtrum, though this could be due to the specialised, beak-like jaws in living species.

In most mammals, the philtrum is a narrow groove that may carry dissolved odorants from the rhinarium or nose pad to the vomeronasal organ via ducts inside the mouth.

For humans and most primates, the philtrum survives only as a vestigial medial depression between the nose and upper lip.

The human philtrum, bordered by a pair of ridges known as the philtral columns, is also known as the infranasal depression, but has no apparent function. That may be because most higher primates rely more on vision than on smell. Strepsirrhine primates, such as lemurs, still retain the philtrum and the rhinarium, unlike monkeys and apes.

In humans, the philtrum is formed where the nasomedial and maxillary processes meet during embryonic development. When these processes fail to fuse fully, a cleft lip may result.

A flattened or smooth philtrum may be a symptom of fetal alcohol syndrome or Prader–Willi syndrome.

A study of boys diagnosed with autism spectrum disorders found that a broader than average philtrum is one of a cluster of physical attributes associated with autism.

In Jewish tradition, each embryo has an angel teaching it all of the wisdom in the world while it is in utero. The angel lightly taps the infant's upper lip before birth to prevent the infant from revealing the secrets of the universe; the infant then forgets the Torah it has been taught. Some believers of the story speculate that this is the cause of the philtrum, but it does not have a basis in traditional Jewish texts.