The Thai form of Buddhism is sometimes called Lankavamsa (meaning Singhalese or Sri Lankan lineage) because it was introduced to the 13th century Sukhothai Kingdom by monks from Sri Lanka, at the time that Sukhothai was the capital of what was to become modern Thailand. This form of Buddhism grew as the Sukhothai Kingdom expanded over Northern and Central Thailand.

However, Ayuthaya in central Thailand grew into a powerful kingdom, and eventually annexed Sukhothai in 1376. The Thai Kings of Ayuthaya made conquests over areas of central Thailand formerly held by the Khmer empire, which had been practicing a blend of Buddhism and Hinduism much more akin to Mahayana Buddhism than to the Sri Lankan form. The Ayuthaya kings incorporated many of these beliefs into the Buddhism they had inherited from Sukhothai. This has had a lasting effect on Thai Buddhism to this day.

One very interesting thing about Buddhism in Thailand is that although the King and Royal family are Buddhists, Royal Ceremonies are actually performed according to HINDU tradition, not Buddhist tradition, with Brahmin monks presiding over the ceremonies. There is indeed a family of Brahmins that live in Bangkok for the primary purpose of officiating over these ceremonies and performing rites for the royal family of Thailand. This is primarily due to the original belief that the King of Thailand is a worldly incarnation of the Hindu God Vishnu. Another thing to note is, despite what many people think, Thai Buddhist monks (and most Thai Buddhists) are NOT vegetarians. Indeed, Thai monks are expected to go out in the morning and collect alms from the lay people (which is a way to help lay people make merit to erase their own karma - it is not begging). The monks are then supposed to eat what is given them, feeling neither joy nor sorrow in what food has been placed in their bowl.

It is ironic to note that during Dutch persecution in Sri Lanka during the 18th century, the ordination lineage broke down. It was Thailand (then known as Siam) that restored the Sangha (brotherhood of monks) in Sri Lanka. Hence, the main sect of Buddhism in Sri Lanka is known as Siam Nikaya (meaning Siam Sect).

The Two Main Nikayas (or sects) of Buddhism in Thailand:

The Sangha in Thailand is broken down into two main Nikayas. The first is the Maha Nikai, and the second is the Thammayut Nikai. The later was formed by King Mongut (Rama IV, who most Westerners know from the historically inaccurate "The King and I"). This was patterned after an earlier Mon form of monastic discipline, which had been practised by King Mongut before he ascended the Thai throne.

In general, discipline is stronger for Thammayut Nikai monks, being allowed to eat only one meal a day and only that which is offered to them in their alms bowls. They are also expected to be proficient in both meditation and scripture study. The Thammayut Nikai makes up only about 3% of the total Buddhist Sangha in Thailand.