South Asia is the southern subregion of Asia that is defined in both geographical and ethnic-cultural terms. South Asia, with a population of 2.04 billion, contains a quarter (25%) of the world's population. As commonly conceptualised, the modern states of South Asia include Bangladesh, Bhutan, India, the Maldives, Nepal, Pakistan, and Sri Lanka, with Afghanistan also often included, which may otherwise be classified as part of Central Asia. South Asia borders East Asia to the northeast, Central Asia to the northwest, West Asia to the west and Southeast Asia to the east. Apart from Southeast Asia, Maritime South Asia is the only subregion of Asia that lies partly within the Southern Hemisphere. The British Indian Ocean Territory and two out of 26 atolls of the Maldives in South Asia lie entirely within the Southern Hemisphere. Topographically, it is dominated by the Indian subcontinent and is bounded by the Indian Ocean in the south, and the Himalayas, Karakoram, and Pamir Mountains in the north.

Settled life emerged on the Indian subcontinent in the western margins of the Indus River Basin 9,000 years ago, evolving gradually into the Indus Valley Civilisation of the third millennium BCE. By 1200 BC, an archaic form of Sanskrit, an Indo-European language, had diffused into India from the northwest, with the Dravidian languages being supplanted in the northern and western regions. By 400 BC, stratification and exclusion by caste had emerged within Hinduism, and Buddhism and Jainism had arisen, proclaiming social orders unlinked to heredity.

In the early medieval era, Christianity, Islam, Judaism, and Zoroastrianism became established on South Asia's southern and western coasts. Muslim armies from Central Asia intermittently overran the plains of northern India, eventually founding the Delhi Sultanate in the 13th century, and drawing the region into the cosmopolitan networks of medieval Islam. The Islamic Mughal Empire, in 1526, ushered in two centuries of relative peace, leaving a legacy of luminous architecture. Gradually expanding rule of the British East India Company followed, turning most of South Asia into a colonial economy, but also consolidating its sovereignty. British Crown rule began in 1858. The rights promised to Indians were granted slowly, but technological changes were introduced, and modern ideas of education and the public life took root. In 1947, the British Indian Empire was partitioned into two independent dominions, a Hindu-majority Dominion of India and a Muslim-majority Dominion of Pakistan, amid large-scale loss of life and an unprecedented migration. The 1971 Bangladesh Liberation War, a Cold War episode resulting in East Pakistan's secession, was the most recent instance of a new nation being formed in the region.

South Asia has a total area of 5.2 million km² (2 million sq.mi), which is 10% of the Asian continent. The population of South Asia is estimated to be 2.04 billion or about one-fourth of the world's population, making it both the most populous and the most densely populated geographical region in the world.

In 2022, South Asia had the world's largest populations of Hindus, Muslims, Sikhs, Jains, and Zoroastrians. South Asia alone accounts for 90.47% of Hindus, 95.5% of Sikhs, and 31% of Muslims worldwide, as well as 35 million Christians and 25 million Buddhists.

The South Asian Association for Regional Cooperation (SAARC) is an economic cooperation organisation in the region which was established in 1985 and includes all of the South Asian nations.

The geographical extent is not clear cut as systemic and foreign policy orientations of its constituents are quite asymmetrical. Beyond the core territories of the Indian Empire (territories of the British Empire which were under the system of British Raj), there is a high degree of variation as to which other countries are included in South Asia. There is no clear boundary – geographical, geopolitical, socio-cultural, economical, or historical – between South Asia and other parts of Asia, especially Southeast Asia and West Asia. Most of this region is peninsular, resting on the Indian Plate and being isolated from the rest of Asia by mountain barriers; it resembles a diamond which is delineated by the Himalayas on the north, the Hindu Kush in the west, and the Arakanese in the east, and which extends southward into the Indian Ocean with the Arabian Sea to the southwest and the Bay of Bengal to the southeast.

The common definition of South Asia is largely inherited from the administrative boundaries of the Indian Empire, with several exceptions. The current territories of Bangladesh, India, and Pakistan which were the core territories of the British Empire from 1857 to 1947 also form the core territories of South Asia. The mountain countries of Nepal and Bhutan, two independent countries that were not under the British Raj but were protectorates of the Empire, and the island countries of Sri Lanka and the Maldives are generally included. By various definitions based on substantially different reasons, the British Indian Ocean Territory and the Tibet Autonomous Region may be included as well. Myanmar (Burma), a former British colony and now largely considered a part of Southeast Asia, is also sometimes included. Afghanistan is also included by some sources.