Theravāda (/ˌtɛrəˈvɑːdə/; lit. 'School of the Elders'; Chinese: 上座部佛教; Vietnamese: Thượng tọa bộ) is Buddhism's oldest existing school. The school's adherents, termed Theravādins (anglicized from Pali theravādī), have preserved their version of the Buddha's teaching or Dhamma in the Pāli Canon for over two millennia.

The Pāli Canon is the most complete Buddhist canon surviving in a classical Indian language, Pāli, which serves as the school's sacred language and lingua franca. In contrast to Mahāyāna and Vajrayāna, Theravāda tends to be conservative in matters of doctrine (pariyatti) and monastic discipline (vinaya). One element of this conservatism is the fact that Theravāda rejects the authenticity of the Mahayana sutras (which appeared c. 1st century BCE onwards). Consequently, Theravāda generally does not recognize the existence of many Buddhas and bodhisattvas believed by the Mahāyāna school, such as Amitābha and Vairocana, because they are not found in their scriptures.

Theravāda derives from Indian Sthavira nikāya (an early Buddhist school). This tradition later began to develop significantly in India and Sri Lanka from the 3rd century BCE onwards, particularly with the establishment of the Pāli Canon in its written form and the development of its commentarial literature. From both India, as its historical origin, and Sri Lanka, as its principal center of development, the Theravāda tradition subsequently spread to Southeast Asia, where it became the dominant form of Buddhism. Theravāda is the official religion of Sri Lanka, Myanmar, and Cambodia, and the main dominant Buddhist variant found in Laos and Thailand. It is practiced by minorities in India, Bangladesh, China, Nepal, North Korea, Vietnam, the Philippines, Indonesia, Malaysia, and Taiwan. The diaspora of all of these groups, as well as converts around the world, also embrace and practice Theravāda Buddhism.

During the modern era, new developments have included Buddhist modernism, the Vipassana movement which reinvigorated Theravāda meditation practice, the growth of the Thai Forest Tradition which reemphasized forest monasticism and the spread of Theravāda westward to places such as India and Nepal, along with Buddhist immigrants and converts in the European Union and in the United States.

The Theravāda school descends from the Vibhajjavāda, a division within the Sthāvira nikāya, one of the two major orders that arose after the first schism in the Indian Buddhist community. Theravāda sources trace their tradition to the Third Buddhist council when elder Moggaliputta-Tissa is said to have compiled the Kathavatthu, an important work which lays out the Vibhajjavāda doctrinal position.

Aided by the patronage of Mauryan kings like Ashoka, this school spread throughout India and reached Sri Lanka through the efforts of missionary monks like Mahinda. In Sri Lanka, it became known as the Tambapaṇṇiya (and later as Mahāvihāravāsins) which was based at the Great Vihara (Mahavihara) in Anuradhapura (the ancient Sri Lankan capital). According to Theravāda sources, another one of the Ashokan missions was also sent to Suvaṇṇabhūmi ("The Golden Land"), which may refer to Southeast Asia.

By the first century BCE, Theravāda Buddhism was well established in the main settlements of the Kingdom of Anuradhapura. The Pali Canon, which contains the main scriptures of the Theravāda, was committed to writing in the first century BCE. Throughout the history of ancient and medieval Sri Lanka, Theravāda was the main religion of the Sinhalese people and its temples and monasteries were patronized by the Sri Lankan kings, who saw themselves as the protectors of the religion.

Over time, two other sects split off from the Mahāvihāra tradition, the Abhayagiri and Jetavana. While the Abhayagiri sect became known for the syncretic study of Mahayana and Vajrayana texts, as well as the Theravāda canon, the Mahāvihāra tradition did not accept these new scriptures. Instead, Mahāvihāra scholars like Buddhaghosa focused on the exegesis of the Pali scriptures and on the Abhidhamma. These Theravāda sub-sects often came into conflict with each other over royal patronage. The reign of Parākramabāhu I (1153–1186) saw an extensive reform of the Sri Lankan sangha after years of warfare on the island. Parākramabāhu created a single unified sangha which came to be dominated by the Mahāvihāra sect.