The Atiratra Agnicayana (ati-rātrá agní-cayana "the building up of the fireplace performed over-night") or Athirathram (Malayalam: അതിരാത്രം); the piling of the altar of Agni is a Śrauta ritual of the Vedic religion, the predecessor of modern day Hinduism which is considered to be the greatest ritual as per the Vedic ritual hierarchy. It is also the world's oldest surviving ritual. Its mantras and theological explanations in the Brahmana texts are first attested in the Yajurveda Samhitas (Taittiriya, Kathaka; Vajasaneyi). The practice of this ritual was generally discontinued among Brahmins by the late Vedic period, during the rise of Jainism and Buddhism in India. Nevertheless, a continuous, unbroken 3,000 year tradition has been found to exist among a few Nambudiri Brahmin families in Kerala, South India.

The entire ritual takes twelve days to perform, in the course of which a great bird-shaped altar, the uttaravedi "northern altar" is built out of 10,800 bricks. The liturgical text is in chapters 11 to 18 of the White Yajurveda; the corresponding exposition of the ritual is in Books 6 to 9 of the Shatapatha Brahmana.A total of 29 sastra's and stuti's from Rigveda and Samaveda are also recited in entire Agnicayana rite.. The original essence and purpose of the ritual is not correctly known. But, the immediate practical purpose of the Agnicayana is to build up for the sacrificer an immortal body that is permanently beyond the reach of the transitoriness, suffering, and death that, according to this rite, characterize man's mortal existence.