Mainland Southeast Asia 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 Peninsular Malaysia.

Southeast Asia is the geographical south-eastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and north-west of mainland Australia which is part of Oceania. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. Apart from the British Indian Ocean Territory and two out of 26 atolls of Maldives in South Asia, Maritime Southeast Asia is the only other subregion of Asia that lies partly within the Southern Hemisphere. Mainland Southeast Asia is entirely in the Northern Hemisphere. East Timor and the southern portion of Indonesia are the parts of Southeast Asia that lie south of the Equator.