Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainland 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 completely in the Northern Hemisphere. Timor-Leste and the southern portion of Indonesia are the only parts in Southeast Asia that are south of the Equator.

The region lies near the intersection of geological plates, with both heavy seismic and volcanic activities. The Sunda Plate is the main plate of the region, featuring almost all Southeast Asian countries except Myanmar, northern Thailand, northern Laos, northern Vietnam, and northern Luzon of the Philippines, while the Sunda Plate only includes western Indonesia to as far east as the Indonesian province of Bali. The mountain ranges in Myanmar, Thailand, Peninsular Malaysia and the Indonesian islands of Sumatra, Java, Bali, Lesser Sunda Islands, and Timor are part of the Alpide belt, while the islands of the Philippines and Indonesia as well as Timor-Leste are part of the Pacific Ring of Fire. Both seismic belts meet in Indonesia, causing the region to have relatively high occurrences of earthquakes and volcanic eruptions, particularly in the Philippines and Indonesia.

It covers about 4,500,000 km² (1,700,000 sq mi), which is 8% of Eurasia and 3% of Earth's total land area. Its total population is more than 675 million, about 8.5% of the world's population. It is the third most populous geographical region in Asia after South Asia and East Asia. The region is culturally and ethnically diverse, with hundreds of languages spoken by different ethnic groups. Ten countries in the region are members of the Association of Southeast Asian Nations (ASEAN), a regional organisation established for economic, political, military, educational, and cultural integration amongst its members.

Southeast Asia is one of the most culturally diverse regions of the world. There are many different languages and ethnicities in the region. Historically, Southeast Asia was significantly influenced by Indian, Chinese, Muslim, and colonial cultures, which became core components of the region's cultural and political institutions. Most modern Southeast Asian countries were colonized by European powers. European colonisation exploited natural resources and labour from the lands they conquered, and attempted to spread European institutions to the region. Several Southeast Asian countries were also briefly occupied of the Japanese Empire during World War II. The aftermath of World War II saw most of the region decolonised. Today, Southeast Asia is predominantly governed by independent states.