Officially introduced in the 2nd century BC by Venerable Mahinda of South India to King Devanampiyatassa, Buddhism is the religion followed by the majority of Sri Lankans. Sri Lanka is the world's oldest Buddhist country, with the continuing practice of the religion throughout the millennia.

Unlike Myanmar, Japan, China, Thailand and most other countries following the Mahayana version of Buddhism, Sri Lanka is the current centre of Theravada Buddhism. The obvious difference between the two versions of Buddhism is, Mahayana Buddhism focuses on Aamisa Pooja, the offering of worldly material to the Buddha, and Theravada Buddhism focuses on Prathipaththi Pooja, the practice of the dhamma philosophy.

Strictly speaking, because of many religious influences through centuries of international relations, the Buddhism followed by the locals in Sri Lanka has a slight mixture of the Mahayana Buddhism, but the practice of the ordained Buddhist monks are that of the pure Theravada.

The historic incidents leading to the current state of Buddhism in Sri Lanka is of great interest. Emperor Ashoka, of the Gaandhar generation, sent his venerated son, the most venerable Mahinda, with the message of Buddhism, to meet the then ruler of Ceylon, King Devanampiyatissa. The local king, convinced of the dhamma's qualities, arranged for a variety of Dhamma sessions to be held in the kingdom, thus spreading the religion throughout the country of the time.

The next visit of significance to the religion was made by Sanghamitta, a Buddhist nun and sister of most venerable Mahinda. The sacred right branch of the Bodhi tree, under which the Lord Buddha attained Enlightenment, was brought to Sri Lanka and planted in Anuradhapura, where it stands to date. This sacred tree is worshipped and glorified by the large amounts of Buddhists making daily pilgrimages to the site.

The Buddhist monks of Sri Lanka played a main role in spreading the Buddha Dhamma throughout South Asia. The Thripitaka, the three scriptures containing all the details of the Dhamma preached by Lord Buddha, was written in palm manuscripts at Aloka Viharaya (the illuminated temple) during King Vatta Gamini's reign in ancient Ceylon. The Aluviharaya in Matale displays the incident through statues depicting the writing.

Beginning from the temples and shrines built under the supervision of most venerable Mahinda, Sri Lanka has a vast collection of temples, stupas and monk abodes all over the country. Most known areas for ancient Buddhist temples and religious structures are Anuradhapura, Polonnaruwa, Dambulla, Mahiyanganaya, Sri Pada (Adam's Peak) and Kandy. All of these ancient religious sites contain a stupa, a dome-like structure with a smaller box shape on top of the exterior of the dome and then a conical shape over the square. These stupas are believed to be housing the sacred relics of Lord Buddha, including locks of hair, bones and teeth. The most important sacred relic available in Sri Lanka is the tooth relic, enshrined in Kandy, at the famous Temple of the Tooth. Other stupas include the great Ruwanweliseya, the largest stupa in Sri Lanka, renovated and rebuilt many times over, and Mahiyangana stupa, Dambulla Viharaya, Kelaniya stupa and many more. The Gal Viharaya in Polonnaruwa, the Samaadhi statue and the Avukana statue are among the other religious sites of great importance in Sri Lanka.