Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian peninsula by the Gulf of Mannar and the Palk Strait. It shares a maritime border with the Maldives in the southwest and India in the northwest, and it lies across the Bay of Bengal from Bangladesh and Myanmar in the northeast and the Andaman and Nicobar Islands in the east. Sri Jayawardenepura Kotte is the legislative capital of Sri Lanka, while the largest city, Colombo, is the administrative and judicial capital which is the nation's political, financial and cultural centre. Kandy is the second-largest urban area and also the capital of the last native kingdom of Sri Lanka. The most spoken language Sinhala, is spoken by the majority of the population (approximately 17 million). Tamil is also spoken by approximately five million people, making it the second most-spoken language in Sri Lanka.

Sri Lanka has a population of approximately 22 million and is home to several cultures, languages and ethnicities. The Sinhalese people form the majority of the population, followed by the Sri Lankan Tamils, who are the largest minority group and are concentrated in northern Sri Lanka; both groups have played an influential role in the island's history. Other long-established groups include the Moors, Indian Tamils, Burghers, Malays, Chinese, and Vedda.

Sri Lanka's documented history goes back 3,000 years, with evidence of prehistoric human settlements dating back 125,000 years. Many names have been used to refer to the island, with Ceylon being used post-independence and still in use in some cases. The earliest known Buddhist writings of Sri Lanka, known collectively as the Pali Canon, date to the fourth Buddhist council, which took place in 29 BCE. Also called the Pearl of the Indian Ocean, or the Granary of the East, Sri Lanka's geographic location and deep harbours have made it of great strategic importance, from the earliest days of the ancient Silk Road trade route to today's so-called maritime Silk Road. Because its location made it a major trading hub, it was already known to both East Asians and Europeans as long ago as the Anuradhapura period. During a period of great political crisis in the Kingdom of Kotte, the Portuguese arrived in Sri Lanka and sought to control its maritime trade, with a part of Sri Lanka subsequently becoming a Portuguese possession. After the Sinhalese–Portuguese War, the Dutch colonial empire and the Kingdom of Kandy took control of those areas. Dutch Ceylon was taken by the British Empire, which extended control over the whole island, colonising it as British Ceylon from 1815 to 1948. A national movement for political independence arose in the early 20th century, and in 1948, Ceylon became a dominion. It was succeeded by the republic of Sri Lanka in 1972. Sri Lanka's more recent history was marred by the 26-year Sri Lankan Civil War, which began in 1983 and ended in 2009, when the Sri Lanka Armed Forces defeated the Liberation Tigers of Tamil Eelam.

Sri Lanka is a developing country, ranking 89th on the Human Development Index. It is the highest-ranked South Asian nation in terms of development and has the second-highest per capita income in South Asia. The country has had a long history of engagement with modern international groups; it is a founding member of the SAARC, the G77 and the Non-Aligned Movement, as well as a member of the United Nations and the Commonwealth of Nations.

In antiquity, Sri Lanka was known to travellers by a variety of names. According to the Mahāvaṃsa, the legendary Prince Vijaya named the island Tambapaṇṇĩ ("copper-red hands" or "copper-red earth"), because his followers' hands were reddened by the red soil of the area where he landed. In Hindu mythology, the term Lankā ("Island") appears but it is unknown whether it refers to the island. The Tamil term Eelam (Tamil: ஈழம், romanized: īḻam) was used to designate the whole island in Sangam literature. The island was known under Chola rule as Mummudi Cholamandalam ("realm of the three crowned Cholas").

Ancient Greek geographers called it Taprobanā (Ancient Greek: Ταπροβανᾶ) or Taprobanē (Ταπροβανῆ) from the word Tambapanni. The Arabs and Persians referred to it as Sarandīb (the origin of the word "serendipity") from Sanskrit Siṃhaladvīpaḥ. Ceilão, the name given to Sri Lanka by the Portuguese when they arrived in 1505, was transliterated into English as Ceylon. As a British crown colony, the island was known as Ceylon; it achieved independence as the Dominion of Ceylon in 1948.

The country is now known in Sinhala as Śrī Laṅkā (Sinhala: ශ්‍රී ලංකා) and in Tamil as Ilaṅkai (Tamil: இலங்கை, IPA: [iˈlaŋɡaɪ]). In 1972, its formal name was changed to "Free, Sovereign and Independent Republic of Sri Lanka". Later, on 7 September 1978, it was changed to the "Democratic Socialist Republic of Sri Lanka". As the name Ceylon still appears in the names of a number of organisations, the Sri Lankan government announced in 2011 a plan to rename all those over which it has authority.

The pre-history of Sri Lanka goes back 125,000 years and possibly even as far back as 500,000 years. The era spans the Palaeolithic, Mesolithic, and early Iron Ages. Among the Paleolithic human settlements discovered in Sri Lanka, Pahiyangala (37,000 BP), named after the Chinese traveller monk Faxian; Batadombalena (28,500 BP); and Belilena (12,000 BP) are the most important. In these caves, archaeologists have found the remains of anatomically modern humans which they have named Balangoda Man, and other evidence suggesting that they may have engaged in agriculture and kept domestic dogs for driving game.