The Dhammayuttika Nikaya or Thammayut Nikaya (; ) is an order of Theravada Buddhist monks in Thailand and Cambodia. Its name is derived from the Pali dhamma ("teachings of the Buddha") + yutti (in accordance with) + ka (group).

Founding in Thailand

The Dhammayuttika Nikaya, or simply Thammayut, began in 1833 as a reform movement by Prince Mongkut, son of King Rama II. Thammayut remained a reform movement until passage of the Sangha Act of 1902 formally recognized it as the lesser of Thailand's two Theravada denominations.

Prince Mongkut was a bhikkhu (ordination name: Vajirañāṇo) for 27 years (1824-1851) before becoming the King of Siam (1851-1860); in 1836 he became the first abbot of Wat Bowonniwet. After the then 20-year-old prince entered monastic life in 1824, he noticed what he saw as serious discrepancies between the rules given in the Pali Canon and the actual practices of Thai monks; and aimed to upgrade monastic discipline to make it more orthodox. Mongkut also made an effort to remove all non-Buddhist, folk religious, and superstitious elements that had become part of previous practices. Thammayut monks were expected to eat only one meal a day and the meal was to be gathered during a traditional alms round.

The Thammayut Nikaya has produced two particularly highly revered forest monks: Phra Ajahn Sao Kantasilo Mahathera (1861-1941) and Phra Ajahn Mun Bhuridatta (1870-1949). Their bone fragments were distributed to various people and Thai provinces after the cremation and have since, according to their followers, transformed into crystal-like relics () in various hues of translucency and opacity.

The current Supreme Patriarch of Thailand, Somdet Phra Nyanasamvara Suvaddhana, is a member of the Thammayut Nikaya.

Dhammayuttika Nikaya in Cambodia

In 1855, the Khmer King Norodom invited Preah Saukonn Pan, also referred to as Maha Pan, a Khmer monk educated in the lineage of Thailand's King Mongkut, to establish a branch of the Dhammayuttika order in Cambodia. Maha Pan became the first Supreme Patriarch of the Cambodian Dhammayuttika lineage, residing at Wat Botum Vaddey, a new temple erected by the king specifically for the purpose of housing Dhammayuttika monks. The Dhammayuttika Nikaya in Cambodia benefited from royal patronage, but was also sometimes regarded with suspicion due to its ties to the Thai monarchy.
The Dhammayuttika order in Cambodia suffered greatly under the Khmer Rouge, being particularly targeted due to their perceived ties to the monarchy and a foreign power, in addition to the Khmer Rouge's general repression of the Buddhist hierarchy in Cambodia. Between 1981 and 1991, the Dhammayuttika Nikaya was combined with the Cambodian Mohanikay under a unified sangha system established under Vietnamese auspices. In 1991 King Sihanouk returned from exile and appointed the first new Dhammayuttika sangharaja in ten years, effectively ending the policy of official unification. The Dhammayuttika continues to exist in Cambodia, though its monks constitute a very small minority of the sangha in Cambodia. On issues such as the role of monks in HIV/AIDS treatment and education, its current sangharaja Bour Kry has adopted a more liberal position than that of Mohanikay head Tep Vong, but less radical than that of certain Engaged Buddhist elements of the Mohanikay order.