Tanistry (from Gaelic tana, lordship) was a custom among various Celtic tribes, by which the king or chief of the clan was chosen from among the heads of the septs and elected by them in full assembly. The rules of succession of the dynasty of Alpin of kings of Scotland abided the tanistry rules until 1034.

He held office for life and was required by custom to be of full age, in possession of all his faculties and without any remarkable blemish of mind or body. At the same time, and subject to the same conditions, a tanist or next heir to the chieftaincy was elected, who if the king died or became disqualified, at once became king. Usually the king's son became tanist, but not because the system of primogeniture was in any way recognized; indeed, the only principle adopted was that the dignity of chieftainship should descend to the eldest and most worthy of the same blood.

These epithets, as Hallam says, were not necessarily synonymous, but merely indicated that the preference given to seniority was to be controlled by a due regard to merit (Consul. Hist., vol. iii. c. xviii.). This system of succession left the headship open to the ambitious, and was a frequent source of strife both in families and between the clans. Tanistry was abolished by a legal decision in the reign of James I and the English land system substituted.

The word is preserved in the government of the Republic of Ireland, where the prime minister is the Taoiseach while the deputy prime minister is the Tánaiste.

The concept of the tanist or substitute for the sacred king was taken by by Sir James Frazer and incorporated as a central element in his study of European mythologies, The Golden Bough. Through Frazer the figure of the tanist has appeared in modernist poetry, such as T.S. Eliot's The Waste Land and has influenced Robert Graves' interpretation of The Greek Myths (1955) and The White Goddess. Tanist figures appear in much popular neo-paganism.

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