Buddhism in Southeast Asia includes a variety of traditions of Buddhism including two main traditions: Mahāyāna Buddhism and Theravāda Buddhism. Historically, Mahāyāna had a prominent position in the region, but in modern times, most countries follow the Theravāda tradition. Southeast Asian countries with a Theravāda Buddhist majority are Thailand, Cambodia, Laos, Myanmar, all of them mainland countries.

Vietnam continues to have a Mahāyān majority due to Chinese influence. Indonesia was Theravāda Buddhist since the time of the Sailendra and Srivijaya empires, but Mahāyāna Buddhism in Indonesia is now largely practiced by the Chinese diaspora, as in Singapore and Malaysia. Mahāyāna Buddhism is the predominant religion of Chinese communities in Singapore, Malaysia, Brunei, and Indonesia.

Buddhism reached Southeast Asia both directly over sea from India and indirectly from Central Asia and China in a process that spanned most of the first millennium CE.

In the third century B.C., there was disagreement among Ceylonese monks about the differences in practices between some councils of Bhikkhu monks and Vajjian Monks. The Bhikkhu monks affirmed Theravada traditions and rejected some of the practices of the Vajjian monks. It is thought that this sparked the split between Theravada and Mahayana Buddhism.

Theravada Buddhism was formed and developed by Ceylon Bhikkhus during a period spanning from the third century B.C. to fifth century A.D. Ceylonese influence, however, did not reach Southeast Asia until the eleventh century A.D. Theravada Buddhism developed in Southern India and then traveled through Sri Lanka, Burma, and into Thailand, Cambodia, Laos and Beyond.

In the twelfth century, Mahayana Buddhism developed in Northern India and traveled through Tibet, China and into Vietnam, Indonesia and beyond.

Buddhism is thought to have entered Southeast Asia from trade with India, China and Sri Lanka during the 1st, 2nd and 3rd centuries. One of the earliest accounts of Buddhism in Southeast Asia was of a Theravada Buddhist mission sent by the Indian emperor Ashoka to modern-day Burma in 250 BCE. The mission was received by the Mon kingdom and many people were converted to Buddhism. Via this early encounter with Buddhism, as well as others due to the continuous regional trade between Southeast Asia, China and South Asia, Buddhism spread throughout Southeast Asia. After the initial arrival in modern-day Burma, Buddhism spread throughout mainland Southeast Asia and into the islands of modern-day Malaysia and Indonesia. There are two primary forms of Buddhism found in Southeast Asia, Theravada and Mahayana. Theravada Buddhism spread from India to Sri Lanka then into the region as outlined above, and primarily took hold in the modern states of Burma, Cambodia, Laos, Thailand and southern Vietnam. Mahayana Buddhism is thought to have spread from both China and India during the first and second century into Southeast Asia. Mahayana took root primarily in maritime Southeast Asia, although there was also a strong influence in Vietnam, in part due to their connection with China.

During the 5th to 13th centuries, The Southeast Asian empires were influenced directly from India, so that these empires essentially followed the Mahāyāna tradition. The Srivijaya Empire to the south and the Khmer Empire to the north competed for influence, and their art expressed the rich Mahāyāna pantheon of bodhisattvas.