Mainland Southeast Asia, also known as the Indochinese Peninsula and previously as Indochina, is the continental portion of Southeast Asia east of India and south of China that is bordered by the Indian Ocean to the west and the Pacific Ocean to the east.
The Indochinese Peninsula projects southward from the Asian continent proper. It contains several mountain ranges extending from the Tibetan Plateau in the north, interspersed with lowlands largely drained by three major river systems running in a north–south direction: the Irrawaddy (serving Myanmar), the Chao Phraya (in Thailand), and the Mekong (flowing through Northeastern Thailand, Laos, Cambodia and Vietnam). To the south it forms the Malay Peninsula, on which are located Southern Thailand and Peninsular Malaysia; the latter is variably considered part of Mainland Southeast Asia or separately as part of Maritime Southeast Asia.
Mainland Southeast Asia contrasts with Maritime Southeast Asia, mainly through the division of largely land-based lifestyles in Indochina and the sea-based lifestyles of the Malay and Philippine archipelagos, as well as the dividing line between the Austroasiatic and Sino-Tibetan languages (spoken in Mainland Southeast Asia) and Austronesian languages (spoken in Maritime Southeast Asia).
The countries of mainland Southeast Asia received cultural influence from both India and China to varying degrees. Some cultures, such as those of Cambodia, Laos, Thailand and Malaysia are influenced mainly by India with a smaller influence from China. Others, such as Vietnam, are more heavily influenced by Chinese culture with only minor cultural influences from India, largely via the Champa civilization that Vietnam conquered during its southward expansion.
The region, except for Malaysia, is predominantly Buddhist.