About 88% of the population of Tamil Nadu are Hindus. Muslims and Christians account for 6% and 5.5% respectively. Most of the Christians are Roman Catholics. The majority of Muslims in Tamil Nadu speak Tamil, with less than 40% reporting Urdu as their mother tongue. Tamil Jains number only a few thousand now. Atheist, rationalist, and humanist philosophies are also adhered by sizable minorities, as a result of Tamil cultural revivalism in the twentieth century, and its antipathy to what it saw as Brahminical Hinduism.

The most popular deity is Murugan, also known as Karthikeya, the son of Siva. The worship of Amman, also called Mariamman, is thought to have been derived from an ancient mother goddess, is also very common. Kan̲n̲agi, the heroine of the Cilappatikār̲am, is worshipped as Pattin̲i by many Tamils, particularly in श्रीलंका. There are also many followers of Ayyavazhi in Tamil Nadu, mainly in the southern districts. In addition, there are many temples and devotees of Vishnu, Siva, Ganapathi, and the other Hindu deities.

The most important Tamil festivals are Pongal, a harvest festival that occurs in mid-January, and Varudapirappu, the Tamil New Year, which occurs around mid-April. Both are celebrated by almost all Tamils, regardless of religion. The Hindu festival Deepavali is celebrated with fanfare; other local Hindu festivals include Thaipusam, Panguni Uttiram, and Adiperukku. While Adiperukku is celebrated with more pomp in the Cauvery region than in others, the Ayyavazhi Festival, Ayya Vaikunda Avataram, is predominantly celebrated in the southern districts of Kanyakumari, Tirunelveli, and Thoothukudi.

In rural Tamil Nadu, many local deities, called aiyyan̲ārs, are thought to be the spirits of local heroes who protect the village from harm. Their worship often centers around nadukkal, stones erected in memory of heroes who died in battle. This form of worship is mentioned frequently in classical literature and appears to be the surviving remnants of an ancient Tamil tradition.

The Saivist sect of Hinduism is significantly represented amongst Tamils, more so among Sri Lankan Tamils, although most of the Saivist places of religious significance are in northern India. The Alvars and Nayanars, who were predominantly Tamils, played a key role in the renaissance of Bhakti tradition in India. In the 10th century, the philosopher Ramanuja, who propagated the theory of Visishtadvaitam, brought many changes to worshiping practices, creating new regulations on temple worship, and accepted lower-caste Hindus as his prime disciples.

Christianity is believed to have come to Tamil Nadu with the arrival of St. Thomas the apostle, and the number of Tamil Christians grew during the colonial period. Most Tamil Christians are Catholic and Protestant. Islam started flourishing in Tamilnadu after the arrival of Sulthan Syed Ibrahim Shaheed, descendant of Prophet Muhammmad who came from Madinah, Saudi Arabia during 12th century. His grave is found in Erwadi dargah in Ramanathapuram district. Tamil Muslims are mostly either mainstream Sunni or Sufi.

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