Sri Lanka is the 53rd most populated nation in the world, with the population density being the greatest in western Sri Lanka, especially in and around Colombo. There is a small population on the island of the Wanniyala-Aetto people, also known as Veddahs. These are believed to be the oldest and indigenous ethnic group to inhabit the country.

The Sinhalese form the largest ethnic group, composing approximately 81.9% of the total population. Tamils are concentrated in the North, East, Central and Western provinces of the country. Tamils who were brought as indentured labourers from India by British colonists to work on estate plantations, nearly 50% of whom were repatriated following independence in 1948, are referred to as "Indian Origin" Tamils. They are distinguished from the native Tamil population that has resided in Sri Lanka since ancient times. There is a significant population of 8.0% of Moors, who trace their lineage to Arab traders and immigrants from the Middle East. Their presence is concentrated in the cities and the central and eastern provinces. There are also small ethnic groups such as the Burghers (of mixed European descent) and Malays from Southeast Asia.

Sinhalese and Tamil are the two official languages of Sri Lanka. English is spoken by approximately 10% of the population, and is widely used for education, scientific and commercial purposes. Members of the Burgher community speak variant forms of Portuguese Creole and Dutch with varying proficiency, while members of the Malay community speak a form of creole Malay that is unique to the island.

Although a small island, Sri Lanka enjoys significant religious diversity. Approximately 69% of Sri Lankans are followers of Buddhism.[Theravada Buddhism is the predominant school, with distinctive sects such as Ramanna Nikaya, Amarapura Nikaya and Siam Nikaya being widely followed. Buddhism in Sri Lanka has been deeply influenced by indigenous faiths and traditions, as well as the influences of prevailing Buddhist schools in South East Asia. The ancient and famous Sri Dalada Maligawa or "Temple of the Tooth" is the principal Buddhist Temple in Sri Lanka, and by tradition houses the Tooth of Buddha, visited every year by millions of pilgrims. There are many other famous religious institutions in Sri Lanka that attract many visitors daily. Hinduism is practiced by 7.9% of the population, mainly from the Tamil community. Christianity is practiced by 7% of the population, especially by Burgher people though most Christians are Sinhalese or Tamils. While most Sri Lankan Christians are Catholics, there are also significant numbers who adhere to Dutch Reformed Church and the Anglican Communion. Islam in Sri Lanka is practiced by 8.5% of the population and its adherents are almost entirely Moors and Malays.