Mainland Southeast Asia, also known as Indochina or the Indochinese Peninsula, is the continental portion of Southeast Asia. It lies east of the Indian subcontinent and south of Mainland China and is bordered by the Indian Ocean to the west and the Pacific Ocean to the east. It includes the countries of Cambodia, Laos, Myanmar, Thailand, Vietnam, and the peninsular portion of Malaysia.

The term Indochina (originally Indo-China) was coined in the early nineteenth century, emphasizing the historical cultural influence of Indian and Chinese civilizations on the area. The term was later adopted as the name of the colony of French Indochina (today's Cambodia, Laos, and Vietnam). Today, the term Mainland Southeast Asia, in contrast to Maritime Southeast Asia, is more commonly referenced.