Dhammayuttika Nikāya (Pali; Thai: ธรรมยุติกนิกาย; RTGS: Thammayuttika Nikai; Khmer: ធម្មយុត្តិកនិកាយ, Thommoyutteka Nikay), or Dhammayut Order (Thai: คณะธรรมยุต), is an order of Theravada Buddhist bhikkhus (monks) in Thailand, Cambodia, and Burma, with significant branches in the Western world. Its name is derived from Pali dhamma ("teachings of the Buddha") + yutti (in accordance with) + ka (group).

The order began in Thailand as a reform movement led by a prince who would later become King Mongkut of Siam, before also spreading to Cambodia and Burma. Initially, King Mongkut was frustrated because he could not find monks who understood the original teachings of the Buddha and truly adhered to the rules of the monks. This happened because of the syncretism of Buddhism with Thai folk religion. Consequently, King Mongkut emphasised the use of the Pali Canon as the primary authority for monastic practices, and sought to eliminate all of the syncretic elements. The movement became formally recognised as its own monastic order by the Thai government in 1902, with any Thai Theravada bhikkhus not within the order being referred to as part of the Maha Nikaya.

The Dhammayuttika Nikaya plays a significant political role in Thailand. The order has historically been favored by the Thai government and monarchy, with the order holding the majority of all royal monastic titles in Thailand and most of the Supreme Patriarchs (the head of the Thai monastic community) since its founding having come from the Dhammayuttika Nikaya, despite the order making up less than ten percent of all bhikkhus in Thailand.

Dhammayuttika Nikaya (Thai: Thammayut) began in 1833 as a reform movement led by Mongkut (later King Rama IV), son of King Rama II of Siam. It remained a reform movement until passage of the Sangha Act of 1902, which formally recognised it as the lesser of Thailand's two Theravada denominations, the other being Maha Nikaya.

Prince Mongkut was a bhikkhu (religious name: Vajirañāṇo) for 27 years (1824–1851) before becoming King of Thailand (1851–1868). The then 20 year-old prince entered monastic life in 1824. Over the course of his early meditation training, Mongkut was frustrated that his teachers could not relate the meditation techniques they were teaching to the original teachings of the Buddha. Also, he described what he saw as serious discrepancies between the vinaya (monastic rules) and the actual practices of Thai bhikkhus. Mongkut, concerned that the ordination lines in Thailand were broken by a lack of adherence to this monastic code, sought out a different lineage of bhikkhus with practice that is more in line with the vinaya.

There are several rules in the Theravada monastic code by which a bhikkhu is "defeated" – he is no longer a bhikkhu even if he continues to wear robes and is treated as one. Every ordination ceremony in Theravada Buddhism is performed by ten bhikkhus to guard against the possibility of the ordination being rendered invalid by having a "defeated bhikkhu" as preceptor. Despite this, Mongkut was concerned that the area's lineages of regional traditions were broken. He made every effort to commission a phalanx of bhikkhus in Thailand with the highest probability of an unbroken lineage traceable back to the Buddha.

Mongkut eventually found a lineage among the Mon people in Thailand who had a stronger practice. He reordained in this group and began a reform movement that would become the Thammayut order. In founding the Thammayut order, Mongkut made an effort to remove all non-Buddhist, folk religious, and superstitious elements which over the years had become part of Thai Buddhism. He emphasised the use of the Pali Canon as the primary authority as the basis for monastic practices. Additionally, Thammayut bhikkhus are expected to eat only one meal a day (not two) and the meal was to be gathered during a traditional alms round.

In 1836, Mongkut became the first abbot of the new Wat Bowonniwet Vihara, and it would become the administrative center of the Thammayut order to the present day.