The Siam (also Siyamopali and Siyam) Nikaya is a monastic order within Sri Lanka, founded by Upali Thera and located predominantly around the city of Kandy. It is so named because it originated within Thailand (formerly known in Europe as the "Kingdom of Siam"). The Siyam Nikaya has two major divisions (Malwatta and Asgiriya) and five other divisions within these two major units. The Malwatta and Asgiriya chapters have two separate Maha Nayakas or chief Monks.
On the initiative of Ven. Weliwita Saranankara (1698–1778) the Thai monk Upali visited the Kingdom of Kandy in 1753 during the reign of Kirti Sri Rajasinha of Kandy (1747–1782), and there performed upasampada for a group of Kandyans. The Buddhist monastic order had become extinct thrice during the preceding five hundred years and was reestablished in the reigns of Vimala Dharma Suriya I (1591–1604) and Vimala Dharma Suriya II (1687–1707) as well. These reestablishments were short lived.
Although hagiographies written within Sri Lanka avoid the issue, the foundation of the Siam Nikaya was closely linked to both the aristocratic and caste politics of its era, including an attempted coup d'état that is unusually well-documented, due to the interaction of the colonial Dutch and the king of Kandy at the time:
[T]he plot of 1760... occurred during the reign of Kirti Sri Rajasimha and shortly after the formal beginnings of the Siyam Nikaya in 1753. One group within the local aristocracy conspired to overthrow the king and to place a Siamese prince on the throne. The leaders are said to have included not only key lay administrators... but also some of the leading Siyam Nikaya monks. Valivita Saranamkara, founder of the Siyam Nikaya, and his chief student... were named among the conspirators. The plot was discovered, the Siamese prince deported (with the reluctant assistance of the Dutch), and the lay administrators executed.
Upali Thera believed the Buddhist Sangha in Kandy was suffering from a state of corruption, which included the practice of astrology and his efforts were aimed at "purifying" the practices of the monastic order. It was also through the efforts of Upali Thera that the Kandy Esala Perahera came into being. Annually in Kandy there is a celebration which includes a parade in which the focus is the relic of the tooth of the Buddha. This procession was originally focused on honor to Hindu deities, particularly those incorporated into Sri Lankan Buddhism. Upali Thera believed this to be inappropriate in a Buddhist nation, and his influence led to the king declaring that "Henceforth Gods and men are to follow the Buddha".