Mahāyāna and Theravāda Buddhism developed differently across Southeast Asia in terms of their geographic spread, doctrines, institutional history, and cultural impact.

Theravāda Buddhism became the dominant tradition in mainland Southeast Asia—specifically Thailand, Myanmar, Cambodia, and Laos—largely through transmissions from Sri Lanka and the Mon people of southern Myanmar, beginning in the 11th century. This expansion was tied closely to the support of powerful local rulers who favored Theravāda reforms as a means of social and political consolidation.

In these societies, Theravāda emphasized preservation of the earliest Buddhist scriptures (the Pali Canon), monastic discipline, and personal liberation (Arhat ideal).

The tradition blended with indigenous spirit cults, for example, the worship of nats in Myanmar, and became deeply entwined with education and state institutions.

In contrast, Mahāyāna Buddhism established its strongest foothold in Vietnam, due to cultural and political influence from China. Vietnam today mostly follows Mahāyāna practices, though there are overlaps with Theravāda in certain regions.

Mahāyāna Buddhism in Southeast Asia was characterized by a wider variety of texts (including later sutras), a focus on the path of the bodhisattva (enlightenment for the sake of all beings), greater devotion to a pantheon of Buddhas and bodhisattvas, and more flexible, inclusive liturgy and practice.

Mahāyāna also had a historical presence in maritime Southeast Asia (Indonesia, Malaysia, Singapore) mostly through Chinese diaspora communities rather than native conversions.

Key historical differences:

• Early Southeast Asian Buddhism was diverse, including a mix of Mahāyāna, Theravāda, Vajrayāna, and Hindu practices. Over time, especially from the 13th century onward, most regions (except Vietnam) transitioned toward orthodox Theravāda, often following political unification or religious reform backed by monarchies.

• In places like Cambodia, once-major Mahāyāna and esoteric Buddhist traditions receded as Theravāda gained royal patronage; many Mahāyāna practices survived only as elements integrated into local ritual or folklore.

• In the islands (Indonesia), both strands initially flourished—Borobudur (Java) was a center of Mahāyāna Buddhism, while Srivijaya engaged in both, but most native traditions disappeared after the rise of Islam—with only remnants in small communities today.

In summary, Theravāda Buddhism dominates the cultural, religious, and political landscapes of most of mainland Southeast Asia and centers personal liberation and Pali canon monasticism; Mahāyāna survives predominantly in Vietnam and among Chinese communities, with richer pantheistic ritual, diverse scriptures, and emphasis on the bodhisattva path.

These divergent trajectories reflect not only religious choices but also shifting political alliances, foreign influences, and the adaptability of each tradition to local needs and identities.