The Nair (/ˈnɑː.jər/) or Nairs, also known as Nayar ([naːjər]), are a group of Indian castes, described as "not a unitary group but a named category of castes". The Nair include several castes and many subdivisions, not all of whom historically bore the name 'Nair'. These people lived, and continue to live, in the area which is presently the Indian state of Kerala. Their internal caste behaviours and systems are markedly different between the people in the northern and southern sections of the area, although there is not very much reliable information on those inhabiting the north.

Historically, Nairs lived in large family units called tharavads that housed descendants of one common female ancestor. These family units along with their unusual marriage customs, which are no longer practiced, have been much studied. Although the detail varied from one region to the next, the main points of interest to researchers of Nair marriage customs were the existence of two particular rituals - the pre-pubertal thalikettu kalyanam and the later sambandham - and the practice of polygamy in some areas. Some Nair women also practiced hypergamy with Nambudiri Brahmins from the Malabar area.